The UK currently uses a mix of metric and Imperial units for different purposes. Although labelled with the metric equivalent first, products such as milk are frequently sold in multiples of the pint, usually 1, 2, 4 and 6. Litre containers of milk are however, becoming increasingly available. Most pre-packed foods however are now entirely metric, with no reference at all to Imperial weights, and of Britain's supermarkets only Tesco continues to display Imperial information with any prominence, and then only for loose and freshly-packed produce (it may be noted that Tesco controls 30% of the grocery market in the UK, though in Ireland, where Tesco is also the market leader, there is no use of Imperial measurements). Many products – including jam, strawberries, sausages, fresh coffee and quarter-pounder beefburgers – are still commonly sold by the pound or quarter divisions thereof, even though usually only the equivalent metric weight (113 g, 227 g, 340 g or 454 g) is shown on the label.
By law, since 1 January 2000, all loose goods sold by reference to units of quantity have to be weighed and sold using metric units, but traditional units may additionally be displayed as "supplementary indications". The UK government was required by European law to eliminate by 31 December 2009 almost all non-metric units when used for goods and services sold by quantity. However, the Department of Trade and Industry recently announced that, "the Government intends to support the continued use of supplementary indications after 2009 for an indefinite period – this appeared to indicate that the British Government was in favour of obtaining an indefinite derogation on the use of dual metric/Imperial markings. Finally, on 9 May 2007 it was announced that the European Commission had dropped its plans to enforce the abolition of Imperial units. This means that 'supplementary' Imperial indications will be able to continue indefinitely after that date.
Goods and services sold by a description are not covered by weights and measures legislation. Thus, a fence panel sold as "6 foot by 6 foot" is legal, but a pole sold as "50 pence per linear foot" (with no metric price) is illegal. Road signs and speed measurement in cars do not have a schedule to change to metric. On 23 February 2006 Alistair Darling, then Secretary of State for Transport, confirmed, to some applause, on the BBC Question Time programme that the Government had abandoned its previously long-standing plans to convert the UK's 2 million road signs to metric, purely on the grounds of cost.
Non-metric units allowed by UK law for economic, public health, public safety or administrative use from 1 January 2000 are limited to:
Many British roadsigns use non-metric units to measure metric distances. For example, a sign will warn of a hazard in 110 yards, which converts to 100 metres. Such signs were placed to allow easy conversion to metric units if metrication progressed. Other mixes of metric and non-metric measurements are in vehicle fuel consumption and supply. Whilst fuel is sold in litres, miles-per-gallon is the everyday unit used for fuel consumption and news reports still occasionally refer to fuel prices in pounds-per-gallon. On the UK motorway network, signs display distances in miles (often using the character "m" as an abreviation, in conflict with the metric system where "m" is the symbol for metre) and warnings of junctions are given at 100-yard intervals. However, location markers on the hard shoulder and driver location signs carry codes to allow someone to pinpoint their location in the event of a breakdown or emergency are placed every 100 metres.
Other exceptions common to many other countries include aviation, shipping and rail transport – for example, the foot for aircraft altitude, nautical miles and knots – some of which are non-SI units, but accepted for use with SI.
Draught beer and cider are the only goods that may not be sold in metric units in the United Kingdom; the only legal measures for these drinks when sold on draught are ⅓ pint (190 ml) (rarely encountered), ½ pint (284 ml) and multiples of the latter.
Metric is now the system used in the majority of industries. Attempts to metricate clothing and shoe sizes have been unsuccessful. Imperial measures have also recurred in the computer and television markets, such as when describing screen or monitor sizes in advertising. Products that may appear to be Imperial are actually manufactured to metric specifications, using metric drawings and made on metric machines, even if references to Imperial units persist in some areas. The Coopers' trade is one of the exceptions to this rule.
There are currently active opposition movements in the UK, such as the British Weights and Measures Association and the Active Resistance to Metrication (ARM). A recent success for these activists was against York City council's erection of 30 hiking trail signs with kilometre distance markings. The council agreed to cover the offending markings with plastic discs. ARM supporter Peter Rogers said: "Each time we are successful [in getting metric signs changed], it is a small but significant step towards eradicating them from our country. The Imperial weights and measures of this country are part of our traditions and part of our culture. The attempt to impose metric signs is one by stealth and deception and has been going on for many years.
As use of the new system, originally called the "Decimal System", grew through Europe, pressure grew in the UK for decimalisation. The issue of decimalisation of measurement was intertwined in the UK with decimalisation of currency. The idea was first discussed by a Royal Commission that reported in 1818 and again in Parliament by Sir John Wrottesley in 1824. Another Royal Commission was set up 1838 by Chancellor of the Exchequer Thomas Spring Rice and it reported in 1841 that decimal coinage was required first. A third commission advocated in 1853 decimal coinage in the form £ : 10 florin : 100 cent : 1000 mil. The first florins (one tenth of a pound sterling) were struck in 1849 as silver coins weighing 11.3 grams and having a diameter of 28 millimetres. In the London Tavern on Bishopsgate St. 1854 Sir John Wrottesley set up the "Decimal Association" in order to lobby for decimalisation of both measurement and coinage. Sir John met with Gladstone a few days later but was unable to win him to the idea. In 1862, the Select Committee on Weights and Measures favoured the introduction of decimalisation to accompany the introduction of metric weights and measures. A further Royal Commission "on the question of the introduction of metric system of weights and measures" also reported in 1869.
On the legal front, 1864 saw a Private Member's Bill pass which legalised use of the metric system in contracts. However, ambiguous wording of the 1864 law meant that traders who possessed metric weights and measures were still liable to arrest under Acts 5 and 6 William IV c63. The situation was clarified in 1897 following another Select Committee which also recommended that metrication become compulsory by 1899. In 1904, scientist Lord Kelvin led a campaign for metrication and collected 8 million signatures of British subjects. Two years previously an Empire conference decided that metrication should be compulsory across the British Empire. On the opposition side, 1904 saw the establishment of the British Weights and Measures Association for "the purpose of defending and, where practicable, improving the present system of weights and measures". At this time 45% of British exports were to metricated countries. Parliament voted to set up a Select Committee on the matter.
This Select Committee reported in 1907 and a bill was drafted proposing compulsory metrication by 1910, including decimalisation of coinage. The opposition declared that decimalisation of coinage would cost £100m alone. The matter was dropped in the face of wars and depression, and would not be again raised until the 1951 Hudgson Report, the result of yet another Select Committee.
The Hudgson Report recommended compulsory metrication and currency decimalisation within 10 years. It said "The real problem facing Great Britain is not whether to adhere either to the Imperial or to the metric system, but to maintain two legal systems or to abolish the Imperial." The report also recommended that the change should be done in concert with the Commonwealth (former Empire) and the USA. It also pointed out that metric standards were more accurate than Imperial ones, and that the yard and pound should be pegged to definite metric values. This was done by international agreement in 1959 and currently the yard is defined as 0.9144 metres exactly, and the pound as 0.453 592 37 kg exactly. Agreement could not be reached on the pint (and gallon), and this value still differs between the UK and US (the only countries that maintain legal definitions of these units).
In 1965 the Board of Trade and the Confederation of British Industry declared their full support for metrication and decimalisation. Currency decimalisation finally took place on Decimal Day, 15 February 1971, although £1 did not change in value. The Metrication Board was set up in 1969. Unlike its South African and Australian counterparts which had mandatory powers, it only had an "advisory, educational and persuasive role". Metric units have been taught in UK schools since the late 1960s, and certain industries also converted or largely converted decades ago. For example the paper industry converted in 1970, and the construction industry between 1969 and 1972 – although certain products continue to be produced to with reference to Imperial trade names but made using metric dimensions in the factory; for example, a 13 mm thick plasterboard is still often called 'half-inch', even though the measurement is rounded to a convenient metric size and so is now only approximately half an inch thick.
A Commons debate in 1970 on the introduction of compulsory metrication ended in farce. The governing Labour party was then unpopular and the opposition Conservatives revolted on the issue. Examples include:
The press reports on the debate, particularly those of The Daily Telegraph and The Times, were very favourable to the opinions of the Conservatives. Following the debate the projected deadlines for the phased metrication steps were delayed one by one. The original intention of metrication "in concert with the Commonwealth" backfired; Australia, New Zealand and South Africa all completed their metrication processes by 1980, the year that the Metrication Board was abolished as a cost cutting measure. (In contrast, the situation of metrication in Canada resembles that of the UK, except that all road signs were converted in the 1970s.) The last laws which restricted the sale of metricated goods were only removed in 1995; though it is still illegal to sell draught beer in metric units, which in 2002 led to an Austrian-themed pub being asked to stop selling beer by the half litre traditional German steins.
The European Union Units of Measurement Directive as amended by Directive 89/617/EEC required the UK government to pass laws in 1994 finally permitting the sale of goods using metric labelling, while permitting dual measurement. Public reaction to these regulations was negative . Such suspicion of externally-imposed change has long traditions; as Philip Grierson notes, the town of Lincoln paid "lavish" fines in 1201 rather than use government-imposed reformed weights and measures. Steve Thoburn applied to the European Court of Human Rights on the basis that his human rights had been violated but the court decided that no violation had occurred. George Gardiner of the Federation of Small Businesses called (without apparent response) for a civil disobedience campaign. In 1999 further laws were brought metricating the sale of, among other things, fresh fruit. The "Metric Martyrs" were shop owners that were fined for refusing to use metric units and for overcharging in response to metric customer requests.
In August 2005, the European Commission announced it would require Britain to set a legal deadline for the completion of metrication. However, on 9 May 2007, Industry Commissioner Günter Verheugen announced that the European Commission had dropped its plans to enforce the abolition of Imperial measures from 2010. This means that 'supplementary' imperial indications will be able to continue indefinitely after that date. Furthermore, in a letter to the British MEP Ashley Mote dated 5 June 2007, Commissioner Verheugen announced that in relation to the mile and the pint, "the Commission has no intention to endanger the historical and cultural traditions of Member States.
Some 90% of UK exports go to metric countries, and there are costs to business of maintaining two production lines (domestic and exports to the US in Imperial and export in metric). These have been estimated at 3% of annual turnover by the Institute of Production Engineers, and at £1.1bn (1980) per annum by the CBI. Regardless of UK metrication, goods produced in the UK for export to the US would have still been labelled in non-metric units.