Ichthyaetus melanocephalus (ex. Larus melanocephalus)
Mediterranean Gulls are moderately uncommon in Britain with a few breeding, but more passing through or staying for the winter. They often associate with Black-headed Gulls, but can be differentiated by the white tips to the ends of the wings, lack of grey under the wings and the red bill with a black stripe near the tip. In winter, they have a different pattern of black on the head to Black-headed Gulls. The first photo is of an adult in winter plumage and the second is a 2nd winter bird.
1-2: Selsey Bill, 25/09/2006.