This is a really hard one to answer, and certainly not something you'll get a clear answer from google or amazon on. We've gone through the same routes/questions and we're a major SEO player in the UK with Google.
The long and short is that no-one really seems to know or can be sure, some people say yes others say no.
If SEO and search results is a big thing for you, then the best thing to do is create a UK point of presence with proxy servers in UK based data centres. Better yet make them caching proxy servers something like Varnish will do a great job.
I'm going to follow this thread closely to see if you get an answer that we really can follow.
But to be quite honest most places you hear its not a great big deal - its all on whether you can financially accept the risk of loosing rankings if you were to go ahead and do this. I can report Amazon say they're customers haven't reported a loss in search result rankings for being on the cloud but you could also say on the other hand they're bound to be bias and say such things.
Still if its a possible worry and you can afford it (and have enough traffic that it warrants the extra costs) - get a UK set of High Availability Proxy servers (better yet varnish) would be the recommendation for being on the safe side.