https://www.gov.uk/buying-carrying-knives
The laws about buying and carrying a knife depend on the type of knife, your age and your circumstances.
Basic laws on knives
It is illegal to:
sell a knife of any kind to anyone under 18 years old (16 to 18 year olds in Scotland can buy cutlery and kitchen knives)
carry a knife in public without good reason - unless its 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
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 without good reason.
The maximum penalty for an adult carrying a knife is 4 years in prison and a fine of £5,000.
Good reasons for carrying a knife
Examples of good reasons to carry a knife in public can include:
taking knives you use at work to and from work
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 youve got a good reason to carry a knife if youre charged with carrying it illegally.
Banned knives
There is a ban on the sale of some knives:
flick knives (also called switchblades or automatic knives) - where the blade is hidden inside the handle and shoots out when a button is pressed
butterfly knives - 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
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 your local police to check if a knife is illegal.
So if not on the banned list and you are charged with illegal carrying then "a court will decide"