The laws and rules about carrying knives in the UK is very confusing. Especially when you start trying to work out whether multi-tools like the Leatherman tools are legal or not. Most of the larger Lethermans have safety or locking blades and this is where the trouble arises.

It is legal to carry a knife with a folding blade in a public place in the UK provided the blade is under 3 inches and the blade does not lock into place.
It is illegal to carry a knife with a longer blade or a blade that locks, unless you have a reasonable excuse for doing so. Bear in mind that this is the legal definition of reasonable, not necessarily your definition of reasonable, and the onus will be on YOU to prove that your excuse is reasonable.

I believe the confusion arises because the actually law does not make it illegal to carry a locking knife with a blade of 3 inches or less. This is covered by the Criminal Justice Act (1998) Section XI "Miscelleaneous" - Subsection 139 - 142 - "Articles with blades or points and offensive weapons”. The Act itself does not mention locking knives at all, it merely says that it’s OK to carry a "folding pocketknife” provided it’s blade is 3 inches or less.

The problem occurred in a subsequent Crown Court case (Harris vs DPP, 1992) where an over enthusiastic lawyer managed to get the judge to rule that a foldable blade must be "readily and immediately foldable at all times". A locking knife is not “readily and immediately foldable” and hence is the same as a fixed blade for the purposes of the Criminal Justice Act (1988).

Even more unfortunately this was ruling was upheld in a subsequent Court of Appeal judgment in 1998 (Regina vs Desmond Garcia Deegan, 1998) without leave for appeal. The Court of Appeal is the highest court in the land which mean that all other courts must adhere to this ruling. Hence, in the eyes of the law, a locking knife is the same as a fixed blade knife.

Offensive weapon per se

As a side note, in the UK, it is illegal to carry an offensive weapon in a public place without reasonable excuse or lawful authority. So is a pocket knife an offensive weapon? Well, no, actually it's not. To understand why, you need to understand the phrase per se.

PER SE
adverb: By or in itself or themselves; intrinsically:

(source: Oxford Dictionaries)

In this context it means something that has been designed, created or modified in such a way that it's primary purpose is to be a weapon.
A pocket knife has been designed to be a tool for cutting things. These things might be apples (for horses), string, feed bags etc. But it's primary purpose is not as a weapon.

Hence it is not illegal to carry a pocket knife ... provided the blade is under 3 inches and it readily folds.

For more information on what constitutes an "offensive weapon per se" have a look at this article - http://www.inbrief.co.uk/offences/offensive-weapon-possessing.htm

Summary

So the bottom line is this - do not carry a knife with a locking blade unless you are very sure you have a reasonable excuse for doing so. And this most definitely includes multi-tools with locking blades such as the larger Leathermans.

I have to say that, personally, I find it very annoying. Multi-tools are very useful things. As for using locking blades - I have at least one fairly large scar on my fingers cause by a pocket knife collapsing unexpectedly. I'd really like to avoid getting another. Locking knives are actually a safety device, and the original law wasn't intended to cover them. It's unfortunate that the powers that be have decided that it does. Given the current political climate with regards to knives, it's unlikely that this will change in the near future.

Ho hum :-(

Sources

The Criminal Justice Act (1988) - http://www.legislation.gov.uk/ukpga/1988/33/contents

http://www.thebushcraftstore.co.uk/knife-law-advice-13-w.asp

http://www.britishblades.com/forums/content.php?12-The-Law-FAQ

http://www.hrcr.org/safrica/arrested_rights/Harris_DirPubPros.htm

Information on what constitutes an "offensive weapon per se" - http://www.inbrief.co.uk/offences/offensive-weapon-possessing.htm

Two things are infinite: the universe and human stupidity; and I'm not sure about the universe. - Albert Einstein