namealot

Depends a lot on what you do with the name and why you’re registering it..?

If you’re registering to start a business, Worried about them taking your name. No way is 100%..? Trade Mark, company registration, company stationary, Proof of advertising the name, client base, can be better to have lots of customers shows the company is growing (the loss would adversely effect your business) rather than just a few (purely paper exercise) charity donations, letters addressed to your company(anything quotes, estimates request for brochures etc) more you show the business is real, established and trading and known buy the name? the better chance you may stand.

If you’re just registering it because you want to sell it to them you could be on sticky ground..?

Why they have not purchased the co.uk themselves…...

Just because there not visibly using the domain(Could just be using email services etc ) does'nt mean they are not trading…

Trademark route for a better case.. should be 100% (not in reality) Lots of classifications (Even if the words are trademarks, it may be possible for someone to register them in another class.) How well they work when going up against a large corporation is not so clear cut …
Some examples of UK trademarks being used by the non UK trademark holder
Afilias use - about.info –check uk trademark E1216811 ABOUT
Afilias use- address.info - check uk trademark 1443487 ADDRESS
Afilias use- directory.info - check uk trademark 276801 DIRECTORY
There are many others too...?

If its an aged name check its history, Check the other company details as much as possible (helps to know what they can spend and when etc). Check udrp drs cases, Do your homework if you paying a decent sum for it... If its a new registered domain doubt it would be worth all the effort ...