The legal limit appears to be 3 inches on a pocket folding knife, but it is up to the discretion of the police officer involved. As with any potential weapon - including other tools and sharpened objects - the onus is on you to give a reason why you have it.
Locking knives are apparently considered extra offensive by British law, fixed blades you might have a hard time explaining; and as you probably know, automatic and butterfly knives are illegal - see the Law section on the Knives UK site:
Happy hunting -
[This message has been edited by Kallisti (edited 18 July 1999).]