It looks exactly like a Great Blue Heron, from this side of the pond.
Wikipedia says "The great blue heron (Ardea herodias) is a large wading bird in the heron family Ardeidae, common near the shores of open water and in wetlands over most of North America and Central America, as well as the Caribbean and the Galápagos Islands. It is a rare vagrant to coastal Spain, the Azores, and areas of far southern Europe."
Perhaps the British Isles have been floating in a southern direction?