just to clear a few things up...
(according to the crown prosecution service website)
the maxium folding knife length in britain is 3"... (not 3.5 or 4"), carrying anything over this in public is illegal.
automatic, switchbalde/flick knives are completly illegal, there is no excuses
Butterfly knives, (the ones where the handles fold around to expose the blade and create a sold handle are locking knives and are also illegal,
knives which fold open but are then locked by means of a pin are also illegal,
(this doesn't incude swiss army knives as you don't have to press a button to fold the blade away.
it is illegal to carry fixed blade knives, (i.e knives that are solidly attached to the handle with no folding mecanism.
(court cases of Harris (1993) and Deegan (1998), a three inch blade locking folding knife was considered legal to carry.
Unfortunately in these two cases it was held that a knife that could not be closed without releasing a lock was in law a fixed blade knife and hence illegal to carry).
Criminal Justice Act 1988 section 139
it is an offense to carry an article with a blade or sharp point in a public place. A folding pocket knife is not included, so long as the cutting edge is under three inches. In practical terms it is best to take 'cutting edge' as meaning the whole blade, sharp or not...
The position that we start from then is that all fixed blade (sheath knife style) knives, all locking folders and all folders with a blade over three inches long are illegal to carry on a daily basis. This means that a regular Swiss Army Knife, penknife, smokers knife etc are perfectly OK to have on you at all times.
Under section139 (4) it is a defence for the accused to show they have good reason or lawful authority to carry the knife in a public place.
Secondly under section139 (5) it is a defence to prove the knife is carried for work, for religious reasons or as part of national costume.
(e.g chefs can carry knives to and from work in a knife roll, or shiks can wear daggers in turbans).
If you've just bought a set of kitchen knives it's aceptabl to have them... (but it's not aceptable to go to a city for a day out, but kitchen knives and take them clubbing with you, even though you are on your way home and just stopping off... when it's 3am, you're steaming drunk outside the kebab shop with a set of uber sharp kitchen knives you've going to be arrested
if you go camping in the woods, it's acceptable to take a machetti to clear scrub...
(but it's not acceptable to have a 2 foot blade in your habds going through town just cause you happen to have a backpack and a sleeping bag!)
just a note on th public place...
'a public place includes any highway and any other premises to which the public have or are permitted to have access, whether on payment or otherwise'
It is important to note that whereas you may regard your vehicle as an extension of your home, the law does not. Your vehicle is a public place in the eyes of the law. This means that it is inadvisable to leave knives, axes etc in your vehicle unattended. It is not impossible that an over enthusiastic officer might decide to make life difficult for you. So keep them out of sight.