whatson - 8:41 am on Sep 24, 2011 (gmt 0)
If it is not a country specific domain name, e.g. its a .com, then changing the ip address to a different country can affect rankings. If you move it to the uk, then it will likely rank higher in google.co.uk, but probably not so well in the US.
In WMT though you can select what country it is you wish to select for the site. So depending on where you want your visitors to come from, then select the country.
If it doesn't matter what country the traffic comes from, then you likely just want the most traffic you can get. Therefore you would want it located in the US, and I would not suggest changing the host to the UK.