It's an adult Kittiwake, which should have been out in the Atlantic, thinking of heading to the breeding cliffs in a few weeks:
But instead it was blown inland by the Atlantic storms and ended up sheltering next to our bike shed for a few hours before dying overnight (there were droppings under the body, showing it had been there a while when alive).
The grey ear covert and nape indicated that it was still in winter plumage:
But it still had the vivid orange gape and tongue:
Also interesting to have a look at the feet, with their short tarsus and vestigial (almost absent) hind toe that gives them their scientific name of Rissa tridactyla (tridactyla = 'three-toed'):
It was extremely emaciated, with a muscle score of 1 at best. There have been lots of dead and dying seabirds washed up on the south/west coast recently, which have been starving due to the stormy weather preventing them from feeding. So they get weak, and then pushed towards the shore and 'wrecked' when they're dead or dying. It seems clear that this Kittiwake was part of the same event, having been unable to feed at sea and so becoming weak, and then blown about 100 km inland before becoming moribund next to the Thames. Shame.