A keyword rich domain is available with both .com and .co.uk extensions available, my site will be targetting the UK market which domain would you go for?
I prefer to go with .com because most people search with .com and the users can also type your site name easily if it is .com rather than .co.uk
If all you care about is ranking in Google.co.uk then having a .co.uk ccTLD will give you a very slight advantage. I would also recommend hosting it on servers in the UK. If, however, you think at any point in the future you would like to rank in Google.com then I would suggest going with a .com TLD. For now you can use a .com TLD domain and set the Geographic Targeting setting in Google Webmaster Tools to target the UK (this will tell Google though you're a .com you prefer to target Google.co.uk visitors). If later you want to rank in Google.com and target US visitors or vistors worldwide then you can just un-set your Geographic Targeting setting in Google's Webmaster Tools.
if you target UK market then co.uk will be the right choice. not only it will help you in terms of ranking on google UK but UK visitors are more likely to click on a UK domain thinking they will get more relevant result
What is the view on getting the .com but keeping the content under /uk?, e.g. everything would be under www.wibble.com/uk/ with a redirect from /. That way if you wanted to target other countries in future you could simply add /usa or whatever. Also people from the UK would see /uk in the URL and perhaps be more likely to click, so best of both worlds?
I would suggest getting both at the same time as you can never tell what may happen in the future and you go to a bigger market.
If you're targeting only the uk market, go with the .co.uk registration. But yuou may want to 'corner up' the .com domain also just in case somebody else registers it and starts competing with you.
I was in a similar position so bought both. My .com redirets to the .co.uk domain and i still appear on spot 1 searching both google.co.uk and google.com so id get them both and do something similar.
If you are targetting UK audience a .co.uk is better, purely for the reason that if someone searched "UK" the domain TLD is picked up. Also make sure your site is hosted in the UK