If your visitors are mainly from the UK, and your current site is on a .com - I would stronly advise getting the .co.uk as a majority of UK surfers always try and type the .co.uk extension.
How do I know this? I am a boring person with too much time on their hands, and also because I own some of the elite .co.uk domains which get enormous traffic from UK visitors purely expecting the Uk version of a .com site.
hope that makes sense, probably not as it is still early Sunday :)
Shak
An important reason is brand protection. I strongly advised a client last year to secure the .co.uk version of his domain. He didn't want to pay the extra $15 annually that I would have charged him. A competitor of course snapped it up and has now put up a redirect page keyworded to death, SEO'd and probably getting more traffic than my client's .com. He is now considering launching a dispute under Nominet's DRS (dispute resolution service) that charges £750 just to come to the table. Going to the full 3 person panel comes in at £3,000. And this is excluding all one's legal fees.
Last but not least is that many UK search engines (including mine, but in this ultra politically correct anti spamming environment would not dare to even think about mentioning its name) give much higher positions to .co.uk sites and in some cases specifically exclude non-.co.uk sites from their indexes.
For the sake of about 30 cents a week are you seriously going to lay yourself open to possible cyber squatting and lost search engine positions?