Google considers multiple signals when determining the geo target for a website. One signal is the IP address of the hosting server, but there are other factors which include the IPs of the people who visit the site, and most importantly (as far as I can tell) the IPs of the sites LINKING to your site. Check the localization of your link base with Blekko and check where your visitors come from in analytics to help you make a decision about the impact of moving your hosting to the UK. Those are two strong signals to Google about your site's localization. Keep in mind that if your target is the US and you move hosts to the UK you will suffer from longer page load times for US visitors which can impact ranking and conversion.