Personally, I think the answer to "Whats better .co.uk or .uk?" is going to come down to how long a view you want to take.
The couk/uk cctld end user desirability is always going to be UK centric. If a business sells to UK consumers, or the service is specifically targeted at the UK. So if you're asking from the perspective of buying names to sell to end users, you limit your potential pool of prospects (just like with any cctld that google doesn't grant pseudo gtld status).
.UK hasn't really entered the minds of most in the UK, most people in the UK, who aren't domainers, I talk to about domains have no idea that theres anything other than .co.uk. Some even think that just having .uk is a typo (like kite26 said).
Long term IMO .uk will win out. Human nature is shorter (in this context) is better. The gov have been helping the case greatly, moving all governmental services, over to gov.uk, and pushing gov.uk on all marketing. This will help penetrate the consciousness of the UK public. It really //grinds my gears// that the likes of the BBC haven't followed suit yet. When they and other large sites do (ebay.uk rather than ebay.co.uk etc)... this will be the tipping point of public awareness. The ".co" bit of our extension adds nothing, and the majority are looking forward to the end of the "double dot" days.
Whenever we buy names on .co.uk, we always pick up the .uk. Just treat the two as different extensions (like com and net), which are for now "related".
This whole area is where there are lots and lots of opportunities coming our way in the domain space. More so than the new gtlds IMO. Theres also going to be huge scope for "getting in wrong" and getting hit with brand infringement or the UK equiv of UDR/cyber squatting.
There are lots of acquisition strategies we use around this whole .co.uk / .uk debate - if anyone's in London and wants to grab a coffee and chat about this stuff, ping me a message.