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From an official British police web site:

Q589: Are there any legal self defence products that I can buy?

The only fully legal self defence product at the moment is a rape alarm. These are not expensive and can be bought from most local police stations or supermarkets.

There are other self defence products which claim to be legal (e.g. non toxic sprays), however, until a test case is brought before the court, we cannot confirm their legality or endorse them. If you purchase one you must be aware that if you are stopped by the police and have it in your possession there is always a possibility that you will be arrested and detained until the product, it's contents and legality can be verified.
However, accepting there is a lot of concern about street crime, we can try to clarify matters a little by putting forward the following points.

• You must not get a product which is made or adapted to cause a person injury. Possession of such a product in public (and in private in specific circumstances) is against the law.
• There are products which squirt a relatively safe, brightly coloured dye (as opposed to a pepper spray). A properly designed product of this nature, used in the way it is intended, should not be able to cause an injury.
• However, be aware that even a seemingly safe product, deliberately aimed and sprayed in someone's eyes, would become an offensive weapon because it would be used in a way that was intended to cause injury.
• Any products bought from abroad have a greater chance of being illegal.

The above advice is given in good faith, you must make your own decision and this website cannot be held responsible for the consequences of the possession, use or misuse of any self defence product. See Q85 for information on the use of reasonable force.