Answer
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 (non-toxic sprays), however, until a case is brought before the court, we cannot say that they are legal.
We cannot endorse any of these products. If you purchase one of these products you must be aware that if you are stopped by the police and are in possession of one then it is possible that you will be arrested and detained until the product, its contents and legality can be verified. Any of these products bought from abroad will more than likely be illegal and it is advisable not to purchase them.
You must also be aware that any of these products could be classed as an offensive weapon and possession of an offensive weapon in a public place is an offence.