BA - not that cheap but mostly half full nowadays so plenty of stretch room..
Bulgaria Air - reasonably reliable out of Gatwick (I went home with them over xmas), again half full. I am told that the Manchester flight has become pretty woeful
EasyJet - great for Gatwick, goes from the old terminal in Sofia so not a very comfortable departure (little airside facilities, bus to the plane). Full all the time
Try Thomas Cook, got flights with them for £90, flew with them last week to Sofia and back. They were excellent, flight was pretty much empty on the way home so got a whole row of seats to fall asleep on plus managed to blag free board and ski carrige when we got to the airport.