The game has definitely changed in the last couple of years and .co.uks definitely rank better than .coms, in fact I have had to duplicate my 17 year old .com on to my .co.uk since I lost so much UK traffic whilst still doing well outside of the UK.
The most annoying thing is that I sometimes see US .coms in the UK SERPs that don't have a chance in hell in supplying my widgets because we supply them!
Overall the .co.uk SERPs have been in quite a mess for 18-24 months now and I do not see them getting any better.
To test which works best log-out of personalisation, clear your cache etc and then type in a few keywords etc and see what comes up. In my widget sector it is dominated by .co.uks these days.
Er..get the .co <= that is the dotco while you are there..
I don't buy as many TLDs each time I reg' a new domain name as Husky does ( he deals with more countries )..but IMO / IME, the dot co is worth getting..it's too close to the other two that you want, to leave it "hanging" out there..
The UK is definitely ccTLD positive (more ccTLD domains than gTLD domains registered in the market). The ratio is probably about 2.5:1 in favour of .uk. Get the .com as well but make sure it redirects to the .co.uk domain name. Ditto for the .co domain. The .co ccTLD is mainly a US domainer phenomenon but in the last web survey of .co websites I ran, the percentage of websites redirecting to the owner's primary website in a TLD outside of .co was actually higher than that of the percentage of developed .co websites. Registering the .co is probably cheaper than a UDRP action.