Ah there you go then, pretty much as I thought.
Crows half the weight of kites & big buzzards. Crows can fly faster, climb faster and change direction faster.
Kites are definitely agile compared to buzzards.
Can't say I've ever seen Herring gulls mobbing buzzards or kites, black-headed gulls & common gulls perhaps, but again they're smaller. Herring gull is a large gull.
Down here in Bath, you don't see buzzards or kites, it's all gulls. As you head east over Wiltshire, you see more buzzards, then Kites as you near Berkshire.
Buzzards & kites seem to co-exist happily enough whenever I've seen them together. Even sharing the same thermal sometimes.
Can't imagine a buzzard taking a pigeon, pigeon is far too fast. Peregrine falcon is the bird for that.
I've seen Blue tits mobbing magpies. Fiesty little beggars, but no match for a sparrowhawk.