Hi Paul, it depends on the aim of the website. I have a lot of sites and always buy both. The problem you have is, if its a commercial site, most people type .com naturally but those who type .co.uk will go to what may be a competitors site.

IMO its best to alter the name slightly and buy both .co.uk and .com to ensure all customers find you with no problems, then issue a 301 redirect to one of them so all traffic is served by the .co.uk or .com instead of both.