I think the .co.uk market is a lot more stable the new gtlds
It is finding its feet again after nominet launched the .uk extension in June 2014. There was a long period of consultation before .uk was launched and this caused uncertainty from about November 2012 when existing .co.uk domain holders started to battle with nominet to get awarded rights to the new extension. That battle was won (with existing .co.uk owners, in most cases, being awarded a 5 years window to register the .uk from June 2014) but there was nearly 2 years of uncertainty and it certainly did affect sales.
Premium .co.uk domains (with rights to the .uk, or holding the .uk) have continued to sell well. Remember the
$650k sale of furniture.co.uk last year. Visit domainprices.co.uk and select .co.uk to search for sales. You'll also see a lot of domain sales from the domainlore platform. That market is mainly domainers but you do get end users on there sometimes. You can find great bargains on there.
If I was investing a lot of money in a .co.uk then I'd want it to have rights to the .uk version (or be sold the .co.uk together with the .uk). The .co.uk version is still the most popular extension in the uk, followed by .com, and I'd say .uk is now picking up and being used by quite a few new websites. Whether it eventually takes over from .co.uk who knows? The new gtlds are hardly seen at all in the UK.
If you buy a .co.uk that hasn't got the .uk as well (maybe the domain was registered since 10th June 2014, or was a pre June 2014 domain where a domain owner registered the .uk and sold off the .co.uk separately - not a nice thing to do if you haven't warned the buyer about the .uk) then I think it does devalue the domain. So try to buy both if you can, or go for a pre June 2014 .co.uk which has rights to register the .uk.
To register the .uk you'll need a uk service address. I've seen a lot of overseas companies starting to use a uk service address and I think some registrars are starting to offer this service. Best to check the nominet website for more information on this new extension and the terms and conditions.