The original system first.
For a numberplate T123 CAB the AB indicates the location.
The combination of MAN which would have been East Ham is reserved for the Isle of Man. Interestingly the GPO combination was resereved for the Post Office at one time even though the PO mark was for East Sussex.

A London
AA Hampshire
AB Worcestershire
AC Warwickshire
AD Gloucestershire
AE Bristol
AF Cornwall
AG Ayrshire
AH Norfolk
AI Meath
AJ North Riding of Yorks
AK Bradford
AL Nottinghamshire
AM Wiltshire
AN West Ham
AO Cumberland
AP East Sussex
AR Hertfordshire
AS Nairnshire
AT Kingston-upon-Hull
AU Nottingham
AV Aberdeenshire
AW Salop
AX Monmouthshire