If a site has hosting in the UK but domain registration in the US then is it an UK site? In other words, if you type in Google UK for "English sites" then does it show up?
All anyone will have is best guess; I think that where the domain is hosted isn't given much (if any) weight aside from the speed factor that Google announced recently. Most likely things like the .TLD, language the page is in, where the visitor is located, and where the backlinks are coming from make the most difference. For example, if the backlinks are mostly coming from UK specific websites (like .co.uk or other sites with largely UK link profiles), even with a .com domain most likely it will rank higher in the UK for a UK visitor than it would in the USA for a USA visitor.
well - the host doesnt matter much - the domain name matters but not as much as it should. what does matter is the keyword your promoting for example car dealership in the UK that would count as a UK website now if you have a .co.uk google with 95% of the time will show your website only on google.co.uk which mean that people from the UK will get to see your site. though if your trying for a certain city in the uk your .co.uk will not help and you'll have to specify in keywords that your selling your products in a certain location. another way would be to use google local business thing . and list your business and phone and all your contact details there.
The fact of the matter is anyone can put bogus WHOIS information, and you can pick a host anywhere in the world. I guess the more appropriate question would be where are they registered to do business, if at all. You can find this out by doing a corporation search in the respective state (If in the US) or go through the appropriate routes to verify a UK LLC.