We need to add a quick note before we get into which one is best. When we upgrade your site to .uk it isn't a case of .co.uk v .uk because your .co.uk will not stop working after we upgrade you. .co.uk and the .uk will work together but .uk will be your main extension.
Branding, styling and the radio test are all important factors in which the .uk beats .co.uk. But all these reasons pale into insignificance compared to the main point of why your business needs the .uk - it will become the UK - or will always be expected to become - the UKs main domain extension in the coming years.
The reason you chose .co.uk in the first place was probably because it was at the time the UKs main extension - that's why you now need to be on .uk
Why will it become the UKs main platform?
In domain language, the shorter something is - the more valuable it tends to be. Probably because there are less distractions to the core message. It is easier to type, it is easier to see in an advert and it is easier to remember. Branding agencies really like to strip everything down to the basics and promote your key attributes. Another key benefit for your and your customers is that .co.uk is 3 characters shorter than .uk, this is so much easier when it comes to typing your domain into the address bar of any search engine.
For those of us who have used Twitter, trying to cram your point into a single tweet is very hard indeed. Saving a single character these days is so frustrating without damaging the message you are trying to get across, therefore being able to save a whole 3 characters without any loss of your message could be important.
Results from the radio test proved that .uk sounds very professional and more crisp than .co.uk when promoting your brand message across. The .org.uk .co.uk and me.uk are so long winded and the brain latches on to the most common noticeable piece, the "UK" part. If you were to say your web domain address in your mind now with both extensions (.co.uk or .uk), which one sounds better, which one would you believe would be the one to drive your business forward to your customers, Customers are what business is all about and this is where you may lose out if your website is not using the new .uk
Upon hearing someone tell others where their site is and after knowing that other sites are using .uk more and more - an ever growing percentage of them will start to type in the .uk as a primary instinct.