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.