I'm fairly certain this is a Cormorant, not a Shag or any sort of diver, seen swimming on Hampstead Heath ponds. It's looking quite distinctive in its winter plumage and with a bright green eye catching the sunlight - almost exotic compared to the sleeker, blacker summer look. The vicious bill which seems to be almost an extension of the shape of the head marks it out as a Cormorant rather than Shag, amongst other things.
This one has a very white and fluffy neck which I haven't seen in most of the archetype photos, but I assume it's not unnatural. Perhaps it's a juvenile, or partway through changing from one look to another.
