rainborick - 1:33 pm on Oct 10, 2012 (gmt 0)
For Google and Bing/Yahoo!, as you mention, geo-location is largely determined by one of two factors: (1) the presence of a Country Code Top Level Domain Name like .uk, .ca, .fr, etc., or (2) for generic TLDs (.com, .net, .org, .info, etc.) the physical location of the server that hosts the site, based on its IP address. Other factors like the geo-location of the sites linking to yours also play a role, at least in Google. Keep in mind that language is a separate issue and is generally a page-level factor.
In essence, by moving your site to a US-based host, you've sent a strong signal that your site should be seen as being in the US. That's why you're getting traffic from google.com now, and very probably less traffic from the UK. I'd suggest taking petehall's advice and find another UK-based host.