I'm not a lawyer, but I believe it is illegal to booby-trap your house when you are on holiday so that a burglar gets blown up by explosives. I think an angry Frenchman got done for this a few decades ago.
Lawyers are much more aware of the fee-earning potential of human rights cases these days.
The cannister would almost certainly be classed as an offensive weapon in these circumstances, so possessing it would be illegal.
Some judgment would be required, a hammer as part of a carpenter's tool kit is not an offensive weapon, but it is if you have one tucked under the driver's seat of your car.
One might think having this lock is more analogous to the 'under the seat' scenario.
The cannister may come under the Firearms Act, in which cases possessing it at all would be an offence unless you had the appropriate licence.
Administering a noxious substance is also an offence, so if it went off as intended, that would also be illegal.
One of the many assault charges is also a possibility.
There is no defence of 'the thief deserved it'.
Seems to me there are far too many hurdles for this lock to get over to ever make it legal in the UK.