The black headed gull is the commonest gull found in the UK. In the winter if you see a gull with a white head (rather perversely) with just a blurry black spot behind the eye, this is most likely a black headed gull. These birds only have a black head in their summer plumage and in fact even then it's technically dark brown. They are to be found pretty much everywhere, being successful opportunists. More details and pictures of summer plumage at Wikipedia.

The example above was photographed on a beach in Northumberland just last week, so even though it's still Autumn – and a relatively balmy one at that – they are in their winter colours. This is an adult (at least two years) as there is no brown left in its feathers and its beak and legs are a deep red.