Jon, thats quite a broad question. Maybe look at sites such as www.whichhosts.com and visit there site to get a feel for the company. Before signing up however, its worth emailing them and asking about there servers and bandwidth quality. Its also worth askign if they are resellers (if so, with whom) or if they have there own servers. If they have support forums, look for feedback from other clients. The UK has many good hosts, but just as many bad!
I know about 5 or 6 other people who use them on my recommendation and they all say the same now.
Saying that I saw a link to Dixiesys or something similar which are US based and they seemed really good and also read a few good comments about them in this forum. There prices killed 34sp's who I thought were cheap
I've enquired upon the subject of hosting before, and the response has been that unless your site has any mission critical applications, there are no immediate problems to hosting in the USA - where overheads are *far* cheaper and prices more competitive than in the UK. The difference in price is about 100% - which isn't to be sniffed at.