It depends on the knife to be honest.
If it's a fixed blade you should be ok, barring the chance of getting hit for customs charges.
Folding knife, you're on shaky ground.
HMRC have a very broad interpretation of "Gravity Knife" (a banned item) and will seize many knives that are not of that type if they can flick it open with a snap of the wrist.
It's a risk.
If it's a folder, and you can find it in the UK, you might be better off paying the extra.