from my own experience the differences are that the .com will cost more and .co.uk is better for geotargetting when it comes to SEO. Although of course if you are wanting to work with people internationally then go for the .com.
No real difference in targetting/SEO, search engines are very good at allocating content to the right location based on what they find on the site, plus you can always notify Google in Webmaster tools. I've always used a .com for my own site (since 05 without any problems) and have the .co.uk redirected to it.
If I was setting up now I would use the .co.uk and redirect the .com to the .co.uk.
.co.uk domains have a touch more click through for UK audience searching so that would be my reasoning.