Really, it absolutely depends on the domain. .co.uk is a specific niche cc-tld - i.e. for the UK market, so therefore it does have a limited appeal. If it's just 3 random characters, e.g. 0f7.co.uk, then reg fee. If it's a word or an acronym then clearly it's worth more but it depends what it is.
Try to check its value in these market place, ( both are London base ); Sedo Auction Domain Focus Auction - London
I have seen a few cracking co.uk's that havent been registered yet but I havent bothered to register them because I dont think I will ever get anything for them. If you want to register them drop me a pm
It depends in the choice of letters. Letters that can be recalled easily and can be an acronym will fetch higher.