I think they've taken a view on this from their lawyer and given it a run. After all, it's like wandering aorund with a can of paint I suppose.
It's not, technically, a s.1 firearm (technically, a pepper spray is) and it's not an offensive weapon I think. For example, has it been tested as a non-irritant on the eyes? Then there is the issue of intent....the packaging suggests that the intent to use it on a third party is pretty clear.
If it were used in self-defence then I'd suggest that you would almost certainly be OK but if you were stopped and searched I think there would be a fair chance you'd be nicked (on suspicion, which would be perfectly valid) or the item confiscated for examination by some coppers. After all, it might be a pepper spray disguised as something else (has happened) and in the current CT environment wandering about with a dodgy chemical-looking device in your bag is going to attract comment. FFS, I wouldn't travel through an airport with one!
So the maunfacturers might have satisfied themselves that it's legal, and (hey) it might be, but I'm also saying you need to be careful and expect a copper to raise their eyebrows if they see it. And like most legal answers, it doesn't properly solve the question.
V!