This bird of prey is "mantling" to hide its prey from other predators, spreading its body and wings around it so it can't be easily seen, especially by other birds.

I'm afraid I'm not quite sure what type of hawk or falcon this is, and indeed I had to look up the official difference between hawks and falcons. Apparently falcons are a subtype of hawks and are generally smaller, faster and take their prey in the air. This particular example was photographed as part of a falconry display at Thorp Perrow Arboretum, so it's not necessarily a native British species. They did say what it was but I think I missed that bit as I tried to placate a crying baby!