The CAA licensing system is based on the numbers generated by what was the original CAA medical department computer data base which started in the 1970s.
All earlier licence numbers were changed over to use the new 6 figure number plus a letter preceded by the licence type. Some PPL holders retained the old 5 figure numbers and for some reason have never moved to the newer data base. This usually implies that the CAA has received no communication from that licence holder to indicate that the licence is still in use (There are over 70,000 such PPLs) I encountered one only this week held by a PPL holder seeking a FAA validation.
The JAA requirement (JAR-FCL 1.075)for the format of a Licence states:
(III) Serial number commencing
with the postal code of the issuing State
and followed by a code of numbers
and/or letters in Arabic numerals and in
Roman script.
The UK has never included the postal code as part of the licence number, (more software required) so it could all change under EASA!