I'm not convinced it was a buzzard that killed the dove. If there were no peregrines or goshawks in the area my money would be on a female sparrowhawk. Why? firstly I believe this bird was killed in the air, there are no signs of attack near or around the deceased,( blood, mass of feathers etc.) so we can assume it wasn't killed where it lay..buzzards cannot (usually) kill birds in flight.....secondly, the removal of the head is charateristic of sparrowhawks & goshawks
Thirdly, but by no means conclusively, a buzzard could easily fly off with a dove should it be disturbed, whereas a sparrowhawk would have more difficulty. I suggested a female sparrowhawk, as being larger, they specailise in hunting larger birds whereas males usually hunt the smaller passerines.
The rabbit skull could well be an old buzard kill, but how it ended the water, who knows...A large.part of it's left zygomatic have been nibbled, but by whom ?
Birds of prey have liquid droppings but eliminate the non digested parts of their meal (fur, feathers, bits of bone) by rejecting, through their beaks, compacted pellets. I cannot tell if the mummified object in the photo is one or not.
The sight of the 2 birds, may have been buzzards, ( sparrowhawks would have been harder to spot due to their speed ), they may even have scared away the original killer & were preparing to eat just before the OP arrived....As we well know, the suspect(s) found at the scene of the crime, is/are not necessarily the perpetrator(s).
Hen harriers arn't completely off my list either.