Which one do you go for? If you pick US and your from the UK why do you pick the US hosting? Also the same for US people if you pick UK hosting why is this?
That would be a strategic and marketing decision. I'm sure experienced UK based forum webmasters would choose UK hosting for ranking and speed reasons.
I normally use UK based companies (Pluto Hosting) because I work there and support from UK companies is also normally better, at least from my experience.
PlutoHost seems to be in the US, as every domain pings to a USA location.
OP - It doesn't really matter, But if your focus is more about stuff in the UK, Or you expect to have a lot more UK visitors then any other country, Then it may be better getting a UK server.
Although limited, site speed is one of Google's points in search engine rankings. I'd much rather a fast local Australian host, then a slower US host, if I had an Australian based website, so I could advance in rankings over some US hosted site. SEOMoz reports that "Geographic Location of the Host IP Address of the Domain" is worth a high 57% of your local rankings (+ccTLD's). That's a pretty high amount, and so having a local host and adding it with Google's site speed ranking factor (about 5% important out of 200+ factors) does matter for ranking and speed.