BX1 1LT is a nongeographic postcode in the United Kingdom, which means it doesn't have a fixed area that it covers. Lloyds TSB uses this particular postcode, as it allows the company some flexibility when it comes to receiving mail. The BX postcode is used by many other companies for the same reason. British military units serving overseas use the BF postcode to indicate the British Forces Post Office.
Most postcodes in the United Kingdom are geographic, so they can be used to find a specific spot on the map. Those starting with EX, for example, are based around the Exeter, Devon, region, whereas those beginning with YO are based in the York area. The number after the area code indicates which subdivision of the region the postcode is based in, and the number and letter combination further subdivides the area. In many cases, It is possible to find the exact street of an address using just a postcode.
Some overseas British territories have postcodes as well to prevent mail going to the wrong place. The Falkland Islands, located south of Argentina, use FIQQ 1ZZ, and Gibralter, located due south of Spain, uses GX11 1AA. However, these are still treated as international post due to the long distances involved.