You do see a fair amount of usage of .UK on a local business level...(side of a van, contact mostly) But these domains have been registered at basic cost as purely a contact, basic web page.
No reasonably sized business is going to throw good money at purely a dot UK presence. despite what Nominet would try to make you believe. Far too much bleed to the .co.uk.
check out 'Domainlore.co.uk' for current selling/bidding prices for both .co.uk and .uk. The only sales in UK over a £100 are for strong single words or towns etc.
The only advantage or should I say leverage, is if you own both the .co.uk and .uk. Most .co.uk buyers would like the UK (for free) if your offering it as a package. otherwise they'll stick with the .CO. UK