For the benefit of non-Brit readers :-
Well it is simple really When the Driver and Vehicle Licensing Agency was running out of numbers for registration plates they changed the format. From March 2001, a new system was started. Plates from that date have two letters, signifying area of registration, two numbers signifying year (but see below) and then three randomly generated letters.
The numbers in 2001 were 01 from March to September when they became 51. So the "1" referred to 2001 unless it was January or February when it was 2002. In March 2002 the numbers changed to 02 and then changed to 52 in September so the "2" referred to 2002 unless it was January or February when it referred to 2003 etc etc. Can't think of any way to make it simpler