Mr R32 M-Spec - I think it'd be confiscated and you'd be fined. Plus with that excuse you'd get done for intent to do battery, not self defence.
I think the best alternative for a battery device which is legal is a small fire extinguisher (chemical type) - you spray the bastards with the chemical and while they're temporarily "blinded" you beat the crap out of them with the canister end. Best of all it's legal because they're legal to carry (Just make sure they're in an easy spot to access.)