what a lot of companies are doing is placing the UK content within the main website. Buy having for example... domain.com/uk and have the domain.co.uk domain point to the location within the main site.
There are good reasons for doing this, it means that all your SEO and marketing efforts are within the one website, as opposed to having to do twice the work by promoting 2 entirely different websites.
Yep I know what you mean. What you can do is have the .com domain point to the homepage and the .com/uk page pointing to UK content. What you could also do is filter users by IP, and suggest setting their preference to the country they are from. This would involve a bit of investment both in terms of data and scripting to make it possible.
If you where to do this effectively .com site would be different depending on where the user comes from. Often when you see a site using this they have an option to change setting (location)