Ok here is the apparent although now not so obvious reasoning behind UK codes. Originally the 'major' airfields were given paired letters the same. ie SS=Stansted, BB=Birmingham AA=Aldergrove CC=Manchester (why?) FF=Cardif etc. JJ=Jersey. The plan was then that airfields close to these would retain the third letter and then have the fourth allocated in a sensible way.
So EGJJ = Jersey JA Alderney, JB Guernsey
EGHH = Bournemouth (Hurn) with its "satellites", HA Compton Abbas, HJ Bembridge, HN Sandown, HR Goodwood, HI Southampton etc. EGP- was reserved for Scotland. I know we have moved away from the 'rules' now but that was the original idea hence KK- Gatwick but it still does not answer why Luton is GW!!
PS
EGTT is teh ICAO designator for London ATCC hence why EGT? is used close to London.