Google is increasingly moving toward localised search, so that if you're searching for a web designer in the UK for example, you'll get very different results than in your searching in the US, even if you don't specify any location in your search.
If you're targeting a UK market you're best off using the .co.uk as your main domain, and then do a 301 redirect from the .com to the .co.uk. Don't have the .com as an Addon domain, as then if people link to that as well you'll miss out on some link love from google. Google's generally clever enough to not penalise both sites for duplicate content, or either, if you have multiple domains serving the same content, but you're still better off with a redirect.
If you're targeting a wolrdwide market you're probably better off with the .com.
You can also target the UK using a .com by simply specifying the target in your header, or in webmaster tools (or both!).
Generally google will figure it out anyway if you're not explicit about your target, using various techniques.
On the subject of location and google rankings, make sure that if you're using a .co.uk, that your server/webspace is located in the UK, as Google will give preference to .co.uk's hosted in the UK (this makes sense, as someone with a .co.uk but based in India for example, shouldn't show up as readily for UK results). A lot of people that get caught out by this are those hosting with 1&1 (where a lot of servers are in Germany), and the big US hosts.
Obviously hosting in your target country also makes sense from customer service and page load speed perspectives.
Besides the technical points, it is also worth considering psychology as has been touched upon. I personally think .com's used to look better, but than .co.uk's are potentially more in favour now. People in the UK probably do click .co.uk's more, especially when they think location will matter (e.g. web design - I'd much rather deal with a designer based in the UK than the US, or a cheap outsource country).
I guess an example of where a .com has a better ring to it/impact would be play.com (I wonder how much they paid for that), but then play.co.uk rhymes
.
Anyway I'm rambling. I hope that helps. Good luck!
