British law has no legal definition of antique. Board of Trade (as was), Customs & Excise, and Home Office guidance to police, long ago settled on '100 years' for most things (there are detail differences for firearms).

With Yale patent monopoly protection expired, the first UK company to make copies of the Yale pin tumbler cylinder was Kenrick in 1908. Legge contemplated production in 1936, but apparently did not start until after WW2. Whether commercial production actually started before 1950 is uncertain. Jim Evans, sometime MD of the key-making company Arthur Hough, had reason to think it did not.

There were over a dozen companies making copies of the Yale cylinder and locks in Britain. They agreed to make them compatible. This was not the case with several rival cylinder locking devices, several of which were lever mechanisms.