As far as I am aware.
It is illegal to carry any knife with a locking blade in the UK, unless you have "good reason" -- which would usually mean you're a farmer in your field cutting things or a fisherman using a heavy line.
I don't have time for a proper google, but this is pretty much how I read the relevant laws last time I found them online:
www.goxplore.net/guides/Knife_law_%28UK%29#Carrying_Knives_in_Public
(Me, I'm not... in any way intoxicated, Fri 30 Sep 2011, 14:04, 2 replies)

Its a shit law
My friend was hiking and was stopped and searched by the police, in his hiking rucksack along with his tent, cooking equipment, sleeping bag etc. they found a lockknife. They arrested him, took him to the station, cautioned him and took the knife. Apparently "I'm hiking/camping and wanted a knife that won't accidentally close on my fingers" is not a "good reason"
( Peej, Fri 30 Sep 2011, 14:22, closed)

Is your friend Ray Mears?

(LetsGetDownToIt, Fri 30 Sep 2011, 15:02, closed)

nope but we went talking less than a 3" blade

( Peej, Fri 30 Sep 2011, 15:17, closed)

Less than 3 inches is safe
Size does matter after all.
(BearWithLittleBrain, Fri 30 Sep 2011, 16:29, closed)

The length is irrelevant, as far as I can tell.
The three inch rule is for pocket knives -- which must fold without any resistance and not be locked open. A lock knife is classed as fixed-blade knife, all of which are illegal to carry without the aforementioned "good reason".
(Me, I'm not... in any way intoxicated, Fri 30 Sep 2011, 17:28, closed)