What self-defense weapons are legal in London?

The link that you cite already states the law: Any “noxious substance” is illegal. So this is likely yucky stuff, but with no irritant. It may be disorienting for a moment, but I doubt it’ll incapacitate a determined attacker.

For practical advice: Don’t. Even if you were a trained officer, going against 4 armed attackers (or even 2) with just a can of pepper spray would be a recipe for disaster. To put it bluntly: You don’t want to anger a person with a knife if you can avoid it.

If you want to feel safer, I’d suggest you look for a reputable self defence class. I assume that there they can teach you how to escape or incapacitate someone if you can’t avoid the confrontation (also, if the spray is any good they’ll probably teach you how to actually use it).

Addition: This answer was to specifically address the legality of pepper spray substitutes (which was the gist of the original question). There is already a great answer on defensive weapons in general, so I'm not going to add to that.

Also, I did not want to suggest that self-defence training would have made a difference in your situation. No type of weapon or training would have.

You may happen to have everyday objects (e.g. an umbrella) that can be of some help against an assailant, even if not in that particular situation.

Carrying pepper spray around may get you into trouble with the law, it is probably illegal despite what vendors may tell you.

See https://en.wikipedia.org/wiki/Self-defence_in_English_law

Self-defence in English law is a complete defence to all non-sexual offences involving the unlawful use of force (i.e. anything from battery to murder). In other words, it results in a charge if a death occurs because minimal force was not used. Generally speaking, the rationale is that the defendant is not guilty of the offence if murder doesn't occur.

and https://en.wikipedia.org/wiki/Offensive_weapon

An offensive weapon is a tool made, adapted or intended for the purpose of inflicting mental or physical injury upon another person.

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Under England/Wales' Prevention of Crime Act 1953, Section 1(1) states it is an offence to carry an offensive weapon on or about the person while in a public place

So far as I can tell, pepper spray is normally considered an offensive weapon.

In Are there any legal self defence products that I can buy? The UK police say

The only fully legal self defence product at the moment is a rape alarm.

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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, its contents and legality can be verified.

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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, if injury does occur, this may be assault.

If you carry anything with the intention of using it against an assailant, it is a weapon. Even walking with your house keys in your hand with a key protruding from your fist.