In England the law states it is illegal to;

• Buy a knife when under the age of 18,
• Carry a knife without good reason, unless it has a folding blade with a cutting edge of less than 3 inches long or less,
• Carry, buy or sell any type of banned knife,
• Use any type of knife in a threatening way (even a legal knife).

 When you can use a knife legitimately;

• If it is used for or being transported to work,
• If it is being exhibited in a museum or gallery,
• If it is being used in a reactment, film, TV, theatre, or for religious purposes,
• If it is to be used for a demonstration.

Ultimately it will be a court that will decide if you have good reason to be carrying a knife, if you are charged.