There are plenty of .org.uk sites, but they're almost all small, UK-only, non-profit organistations.
There is also the fact that many people, myself included, tend to automatically press enter after the org and go to a different site. I always forget the .uk on faw.org.uk for example.
.co.uk is a several-thousand-times-better extension than .org.uk in almost every instance. You are more likely to sell a .co.uk if you're reslleing the name, you are less likely to lose traffic than you would with a .org.uk.
If your site is UK-targeted then you'll be pleased to know that .co.uk is (apparently) favoured above .com in the minds of the UK populace- I'll dig out the link with that info after...