We've mentioned before that if you want to rank well in Google.co.uk (with the UK Filter on) - then it's best to have either a .co.uk extension or be hosted in the UK. Ideally both.

I've recently checked a campaign, which is hosted in the US but has a .co.uk extension. I searched for it in Google.co.uk with the "UK Filter" checked, using the following search to exclude supplementals:

site:www.domain.co.uk -allinurl:www.domain.co.uk

And I noticed that it is not indexed at all in Google.co.uk main index - despite having a .co.uk extension. It is an old domain and indexed fine in Google.com.

So I'm going to update my recommendations in the future to say that if you want to rank well in the UK then you NEED to have BOTH UK hosting and a .co.uk domain.