Increased latency means that transaction based protocols (http, ftp, pop3, imap, smtp, etc) are noticeably slower on anything but a dialup connection.
Hosting a UK site in the US may also have an impact on your search engine placement - for instance, searching for a site with a 'UK site' in Google will yield no results if the site has a GTLD domain and is hosted in the US. Something to bear in mind.
Cost benefits are negligible - for the most part only evident due to massive overselling from the US providers just to be able to compete. It's a very crowded market. Unless you need 50GB of space and 5000GB data transfer for $10/year, I doubt you'll find the US providers any more attractive than their UK counterparts.
As for getting hold of support, if you choose a decent provider, that shouldn't be an issue.
Note that a lot of UK based hosts lease dedicated servers from US based providers for cost reasons.
Last but not least, check out
www.ochostreview.co.uk for some reviews. I'll shortly be adding a small icon showing where the company's servers are located which should prove useful.
