The only thing I can think of is the National Insurance number (NI number)

Thanks!

NI number is probably your best bet, we all get one in the year dot and it stays with us until we croak.
Another thing that might be used now is the NHS number though this has only been used in the last 10 years or so and is only generated if you are registered with the NHS.
Driving license and passport numbers would also be of some use.

Thanks! I figured it had to be something like that. NI it is.

Your local crazed Andy fan here *waves*, police refer to looking up a National Insurance number as "a NIno search" pronounced nine-oh. Police can also search the Electoral Rolls (voters' register list available on computer)town by town. It used to be written on a long piece of paper which got rolled up for storage. The E Rolls rely on people registering themselves and are not greatly up to date, and you have to be over 18 to appear on them though the hard copy has over 16s on it. Scope there for a trip to the archives...

Thanks! This is perfect! Not only for this one, but... well, Andy's been talking to me again...

Ah, Andy has an effect on people...