A Bar-headed Goose!
I have never seen one before, but was familiar with the species. This bird is native to Asia and most likely is an escape or a bird that was dumped into the wild. They have become established in Britain. The bird can't be added to life lists if you follow the rules, but I am not a lister and really don't get into that kind of thing. That being said, I do believe that this bird lives a "wild" life. It was very skittish as compared to the nearby Canada Geese that it later joined up with.
I could see both halluxes, the back toe or "claw". Many breeders cut one off. This bird was not pinioned, as it could fly very well. A while back, one was found in Covington, KY on the Ohio River. It was clearly pinioned and had its right hallux chopped off. Here is a discussion of that bird on Cincybirds:
A very odd bird to say the least. It is still a very nice looking bird with a very funny sound.
For more on the bird in the wild, see here: http://en.wikipedia.org/wiki/Bar-headed_Goose from Wikipedia.