.co.uk is one of the big ccTLDs, it's been around for many years, is a mature extension and firmly rooted into the minds of the UK populace (it's more popular with internet users in the UK than .com).
Be cautious that your nice domain is spelt correctly with proper English spelling (not American English) as this can affect the value- e.g I would not buy honor.co.uk unless I already owned honour.co.uk
Be aware that there is a transfer fee to change owners of .co.uk of £12 (about $23).
You can check a site like namebio.com to see what sorts of .uk domains sell best (here's a selection of the higher amounts show the sort of names that bring decent cash: )
recycle.co.uk $309,901
mobile.co.uk $247,921
fly.co.uk $181,042
taste.co.uk $100,000
4x4.co.uk $75,360
getwired.co.uk $60,001
rugs.co.uk $51,501
businessnetwork.co.uk $41,400
legaljobs.co.uk $39,392
hero.co.uk $39,232
mountainbike.co.uk $36,008
kgb.co.uk $35,600
stationary.co.uk $34,320
ebaby.co.uk $29,089
flirting.co.uk $26,240
rockclimbing.co.uk $25,500