Main bases are MAN, LGW, GLA, BFS.
Manchester is the centre of the universe as far as Airtours are concerned, the rest are considered 'outstations'. Best bases for variety of routes are MAN and LGW (and social functions due to the number of bods), but I found GLA and BHD the friendliest of them all.
Airtours also operate some Summer bases - Birmingham, Bristol, with crewed aircraft, and also fly from Teeside and Liverpool using a W pattern rotation. (i.e. not permanently crewed bases with aircraft).
If you are MAN based, expect to spend a lot of time not exactly at MAN, due to fairly massive undercrewing at all outstations.
Cheers