.org.uk's are great for affiliate sites where ranking it is the key ingredient, but I agree that for anything brandable .co.uk is definetely better, as mentioned .co.uk seems to be people's default setting. I've told countless people before to check out websites with a .org.uk prefix only for them to come back the next day having gone on the .co.uk to say it doesn't work etc.
In terms of ranking I think they all stand on pretty even ground, noticed more .me.uk's ranking recently