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.
Hope that helps,
Greg