Probably the best article on UK numberplates is this one on the Chiltern Vehicle Preservation Group old website that got forgotten...
hereSuggest anyone who likes this makes their own copy as the document ought not be there!As for the plate, a lot depends on the area. Up to about 1925, you will find some cars in the single letter four numbers bracket, but only in areas with few cars. For example, London being a popular area, A7500 was issued in 1904, but I know of one single letter car in another area that was registered in about 1924.
Safest bet would be to go for two letters, and up to four numbers. A real example (my car) is UH8204. That is a Cardiff registration from 1930, but by the mid 30s, the three letter and three number format was commonplace.
The reverse, three numbers and then three letters started in the 1950's although not in some areas where so few cars were registered the earlier format still had plenty of numbers left.
Hope that helps.