I think your best bet, if you can't find anyone in England that can do an American accent, is to do a collaboration with a college student at an American university. If your animation is solid, you shouldn't have difficulty finding somebody.
Thing is, though, I was under the impression that the average Brittish actor found it quite easy to do American. Am I wrong? I think you should try to network with local college actors, first.
Finally, though I've never done animation, I believe it can work either way -- you can do audio first, then match animation, but I think the norm is to do animation first, and match audio.