Basic laws on knives
It is illegal to:

• sell a knife of any kind (including cutlery and kitchen knives) to anyone under 18
• carry a knife in public without good reason - unless it’s a knife with a folding blade 3 inches long (7.62 cm) or less, eg a Swiss Army knife
• carry, buy or sell any type of banned knife (the list of banned knives is below)
• use any knife in a threatening way (even a legal knife, such as a Swiss Army knife)

Lock knives (knives with blades that can be locked when unfolded) are not folding knives, and are illegal to carry in public.

“Public place” (Section 139, subsection 7 of The Criminal justice Act 1988) includes any place to which at the material time the public have or are permitted access, whether on payment or otherwise.
Attention! Your car is defined by law as a public place. A car is not a piece of land and is therefore not private property unless it's parked on private property.

The maximum penalty for an adult carrying a knife is 4 years in prison and a fine of £5,000.

A lock knife for all legal purposes, is the same as a fixed blade knife.
It is perfectly legal to own and use a lock knife on your own property, or on private property where you have the landowners permission. It is, however, illegal to carry a lock knife in a public place, unless you have a good reason to do so.

Good reasons for carrying a knife

According to section 139, subsections 4&5 of The Criminal justice Act 1988....

(4) It shall be a defence for a person charged with an offence under this section to prove that he had good reason or lawful authority for having the article with him in a public place.
(5) Without prejudice to the generality of subsection (4) above, it shall be a defence for a person charged with an offence under this section to prove that he had the article with him—

• for use at work;
• for religious reasons; or
• as part of any national costume.

Examples of good reasons to carry a knife in public can include:

• taking knives you use at work to and from work
• you’re taking knives to a gallery or museum to be exhibited
• the knife is going to be used for theatre, film, television, historical reenactment or religious purposes (eg the kirpan some Sikhs carry)

A court will decide if you’ve got a good reason to carry a knife if you’re charged with carrying it illegally.

There is no exhaustible list defined in law of what constitutes a good reason. If you think you have a good reason and a police officer disagrees, it'll be up to the courts to decide your fate.

Self defence isn't a good reason. If you are carrying a knife for self defence, by definition you are carrying the knife as a weapon, and it is illegal.

Knives that are illegal

It is completely illegal to sell, make, construct, gift or lend one to another individual the banned items. You can legally own banned items in your own home.

Banned items list

Blowpipe or blowgun
butterfly knives (or balisong) - where the blade is hidden inside a handle that splits in two around it, like wings; the handles swing around the blade to open or close it
Knuckleduster
Telescopic truncheon
flick knives (also called ‘switchblades’ or ‘automatic knives’) - where the blade is hidden inside the handle and shoots out when a button is pressed
disguised knives – eg where the blade is hidden inside a belt buckle or fake mobile phone
gravity knives
sword-sticks
samurai swords (with some exceptions, including antiques and swords made to traditional methods before 1954)
hand or foot-claws
push daggers
hollow kubotan (cylinder-shaped keychain) holding spikes
shuriken (also known as ‘death stars’ or ‘throwing stars’)
kusari-gama (sickle attached to a rope, cord or wire)
kyoketsu-shoge (hook-knife attached to a rope, cord or wire)
kusari (weight attached to a rope, cord or wire)
This is not a complete list of banned knives. Contact local police to check if a knife is illegal or not.

Antique weapons, which are defined as weapons over 100 years old at the time of an alleged offence, are excluded.

Link to Knives Act