If I was using a brandable domain for a site, and hadn't got the pair, then spending a few hundred quid on a UK Trade Mark is probably the best protection I could get against any later "pitch invasion" of my brand; valid for up to 20 years if you renew it. A TM would also, to varying degrees, potentially devalue the other one of the pair by limiting it's use.
Personally, I wouldn't start a brand if I only had the .co.uk, but I would with just the .uk.
Generics not so much use here.