Oxford Blue - Spitoon`s description is pretty accurate particularly regarding the third letter.
Just to put you straight about your deduction that there is a geographical and alphabetical link the "P" in the Scottish indicators refers to Prestwick EGPK.
The "N" in the northern ones refers to the now expired Preston Air Traffic Control Centre whose indicator was EGNN.
The "S" in the Eastern English ones refers to Stansted EGSS.
There are a series of military aerodromes having the third letter either "D" "W" "U" "V" and "X" of which Northolt is one.
The "T" in Dunsfold`s referred to EGTT the London Terminal Control Centre.
Hope you`re still awake
