In the UK the third letter designated the circuit therefore GW for Luton and GD for bristol Lulsgate and for most (all?) civilian airfields in Scotland having P, Q for the military
Showing your age now, Lon . . . .
Luton used to be EGLN when connected to Heathrow Comms Centre, then became EGGW when connected to the (then) 'new' Croydon Comms Centre (went there once, Jeeez!) Now everyone is (sort of) connected to Heathrow again. Happy days.
