I believe there is one dynamic campaign for the RAF, RAAF, RNZAF, and RN. There are also single stand alone missions for the RAF - you can also fly Spit Mk. Vs for the Russians in some missions if that's what you're after :).

IL-2 is very focused on the Eastern/Pacific fronts in terms of its campaigns and missions, hence the small number of campaigns relative the USSR/Germany/etc ... for the British and the focus of even the British campaigns being on the Pacific front. However, there are a lot of campaigns - even from a lot of the smaller countries that fought in WW2 that don't usually get represented. Once the expansions are combined there are quite a few more campaigns than there were in the original game:

[url=http://en.wikipedia.org/wiki/IL-2_Sturmovik_(video_game)#Sequels]http://en.wikipedia.org/wiki/IL-2_Sturmovik_(video_game)#Sequels[/url]

The makers of IL-2 did recently come out with a new game called IL-2: Cliffs of Dover (however, it is a new game with a new game engine, so not an expansion of the earlier IL-2 games) which covers the Battle of Britain era. Unfortunately early reviews indicate the game was released in a very buggy state. I don't know the current level of stability of the game and how the patching is going since I don't own it myself.