Pictured above in the weak winter sun is a Black headed gull on a fence post by one of the ponds on Hampstead Heath. In winter these gulls rather unhelpfully have white heads with just a fuzzy black spot on the side. Technically even in the summer it's dark brown, not black, but I can't say I'd notice the difference. This is the commonest inland gull in the UK, so if you see a smallish gull away from the coast then it's probably one of these.