The first standard public telephone kiosk introduced by the United Kingdom Post Office was produced in concrete in 1920 and was designated K1 (Kiosk No.1). This design was not of the same family as the familiar red telephone boxes.
The red telephone box was the result of a competition in 1924 to design a kiosk that would be acceptable to the London Metropolitan Boroughs which had hitherto resisted the Post Office's effort to erect K1 kiosks on their streets.
The Royal Fine Art Commission was instrumental in the choice of the British standard kiosk. Because of widespread dissatisfaction with the GPO's design, the Metropolitan Boroughs Joint Standing Committee organised a competition for a superior one in 1923, but the results were disappointing. The Birmingham Civic Society then produced a design of its own — in reinforced concrete — but it was informed by the Director of Telephones that the design produced by the Office of the Engineer-in-Chief was preferred; as the Architects’ Journal commented, 'no one with any knowledge of design could feel anything but indignation with the pattern that seems to satisfy the official mind.' The Birmingham Civic Society did not give up and, with additional pressure from the Royal Institute of British Architects, the Town Planning Institute and the Royal Academy, the Postmaster General was forced to think again; and the result was that the RFAC organised a limited competition.
The organisers invited entries from three respected architects and, along with the designs from the Post Office and from The Birmingham Civic Society, the Fine Arts Commission judged the competition and selected the design submitted by Giles Gilbert Scott. The invitation had come at the time when Scott had been made a trustee of Sir John Soane's Museum — his design for the competition was in the classical style, but topped with a dome reminiscent of Soane's self-designed mausoleum in St Pancras' Old Churchyard, London. (The original wooden prototypes of the entries were later put into public service at under-cover sites around London. That of Scott's design is the only one known to survive and is still where it was placed all those years ago, in the entrance arch to the Royal Academy.)
The Post Office chose to make Scott's winning design in cast iron (Scott had suggested mild steel) and to paint it red (Scott had suggested silver, with a "greeny-blue" interior) and, with other minor changes of detail, it was brought into service as the Kiosk No.2 or K2. From 1926 K2 was deployed in and around London and the K1 continued to be erected elsewhere.
K3, introduced in 1929, again by Gilbert Scott was similar to K2 but was constructed from concrete and intended for nationwide use. Cheaper than the K2, it was still significantly more costly than the K1 and so that remained the choice for low-revenue sites. The standard colour scheme for both the K1 and the K3 was cream, with red glazing bars.
K4 (designed by the Post Office Engineering Department in 1927) incorporated a post box and machines for buying postage stamps on the exterior. Only 50 kiosks of this design were built.
K5 was a plywood construction introduced in 1934 and designed to be assembled and dismantled and used at exhibitions.
The K6 was the most prolific kiosk in the UK and its growth, from 1935, can be seen from the BT archives:
Prior to these changes, the Tudor Crown had been used in all parts of the United Kingdom, and the British Empire.
To accommodate the two different Crowns on the K6 kiosks, the fascia sections were henceforth cast with a slot in them, into which a plate bearing the appropriate crown was inserted before the roof section was fitted. (This change happened in 1955 and is a very useful way of dating K6 boxes manufactured thereafter.)
Only 12 remain — most having been replaced with the KX100 making the K8 as rare as the K3.
Upon the privatisation of Post Office Telephone's successor, British Telecom (BT), the KX100, a more utilitarian design, began to replace most of the existing boxes. Some 2000 boxes were given listed status and several thousand others were left on low-revenue mostly rural sites but many thousands of recovered K2 and K6 boxes were sold off. Some kiosks have been converted to be used as shower cubicles in private homes. In Kingston upon Thames a number of old K6 boxes have been utilised to form a work of art resembling a row of fallen dominoes. The KX100 PLUS, introduced in 1996 featured a domed roof reminiscent of the familiar K2 and K6. Subsequent designs have departed significantly from the old style red telephone boxes.
Australia and New Zealand each had their own design of red telephone box, and some examples have been preserved in sensitive or historic sites. A brief and colorful campaign was run to "save" the red telephone box in New Zealand by the Wizard of New Zealand.
Kingston upon Hull was the only area of the UK not under the Post Office monopoly, with telephones being under the control of the Corporation of Hull (city council). In Hull and the surrounding area this meant that the telephone boxes were painted cream and had the crown omitted. The Hull telephone system was subsequently privatised and is now operated by Kingston Communications.