Ah, that old one, you get a different answer from each
SEO 
In my personal experience moving one of my sites from a USA server to a UK server caused it to jump into the 1st page, I also saw a similar jump in Google ratings for a uk targeted .org
There are a lot of elements to ranking and from memory you can also now indicate target country via Google's webmaster tool interface, so perhaps hosting location is not as important as it was for ranking, however, if the target is UK and your site is in one of the major UK DC's then it will most likely be faster if served in the UK than USA. Ping time from my here to my old USA DC is nearly 6 times longer than to my UK DC, even longer still to hostgator. (I know ping time is not the best timing method, but it is good enough for a rough comparison!)
So many factors to consider