They are fairly common in the small chalk streams running through the New Forest and I've seen them in the shallows on the River Itchen.
I used to catch them (with a net) as a kid on the River Slea in Licolnshire along with stone loach. Sadly the Slea suffered a massive fish kill following a pollution incident and, although the EA restocked it with dace, roach and bream I doubt whether they stocked bullheads (they were called 'Miller's Thumb' up there) and loach. So it's a pretty widely distributed fish - or was!!