If you are indexed in Google, you sure should be indexed into Google.co.uk
Results may be different for location based results, but I don't think I have seen this question before.. What phrase (and site) are you ranking for in Google.com that your not in co.uk?
It should index there.. ranking though usually sites tagged with EN-GB as the Language, along with Country TLD seem to rank better, I get more Aussie traffic than UK, my guess is because i have more backlinks from Oz sites
To rank well in any country you should first be specific to that area, the ore links you have from .co.uk sites the higher rank you will be in that region