Hi Gary,
Not sure how many there are, my mammals book gives 12, but there's been at least 2 species splits since then, so at least 14. But there's a lot fewer in northern Britain, only 6 or 7 up here in Northumbs.
The largest up here is Noctule, the smallest Pipistrelle; those two are also the two you're most likely to see.
Pipistrelles are tiny, roughly Blue Tit size or smaller (but with longer wings), and are the commonest bat of all in most parts of Britain.
Noctules are about the size of a Swift, and often fly with them, high over lakes at dusk.
Another reasonably frequent bat is Daubenton's Bat, they almost invariably skim low over water, picking floating insects off the surface.
Michael