I've not been to SeaWorld, or similar styled facilities in the UK, but from what I've seen, I think I'd prefer an Aquarium.
They're not particularly interactive, but you know that when you go there.
They're basically a living museum.
Aquariums are great, they're underwater zoos, they're not supposed to be super dooper fun. They're just interesting and awe-inspiring in many cases, and that's exactly how they're advertised. Whereas SeaWorld is more along the lines of "xD xDDD SHAMU!!!! touch the sting rays!!!!! buy this shamu t-shirt merchandise for like $30!!!!!!"
Maybe it was just the SeaWorld I went to in Florida, but in my opinion, the whole thing was just badly organized, boring, and full of stuffty gimmicks.