In a couple of experiments I conducted, only a few urls mind you, the site on the .co.uk domain with uk hosting worked best for ranking in G.co.uk for UK key phrases. If you include the word UK, and places in the UK, in content and title tags, most search engines can figure out that your content is UK specific.
I guess if i could only change one thing and had loads of different tlds, i would host them in the UK. Just in case G looks at the whois data, use a privacy filter for that.
Hope my rambling helps