Some self defense anti attack sprays are legal in the UK
It depends what the court decides is "noxious".
It's difficult to argue that dye or paint in the eyes or Inhaled into the lungs isnt noxious, and as yet there is no clear case law that marks them aside from other s.5 firearms such as pepper sprays.
There was one many years ago that squirted out a single dose of thick foam at such velocity that it was like being punched and I think that side stepped the regs, but it seemed to disappear from the market as quickly as it came.
The only "self defence" device that is beyond dispite legal to carry in the UK is a rape alarm.