Keef has the history essentially correct.
The groupings were to do with the airfields Parent AFTN Station. Not all places were, or indeed are, connected to the AFTN (Aeronautical Fixed Telecommunications Network) so several geographical "parent" stations with new fangled things like on line TTY were established to deal with Flight Plan Messages, Class 1 NOTAMS etc.
Comms not being what they are today, several airfeilds didn't "fit" the system. These parentless stations were all lumped in with FIR desigantors (for example EGTE), the station being EGTT (LONDON FIR)
Oh, for EVO.. EGUF (RAE Farnborough) became EGLF in it's civilian guise.
hmmm... I'll get me anorak....
Rgds BEX