The law is very specific and is laid out in Section 5(1) (b) of the Firearms Act 1968.
This act prohibits any weapon of any description, designed or adapted for the discharge of any noxious liquid, gas or other substance.
There are companies who claim to offer legal alternatives but if the spray has any incapacitation capability it is a weapon. Whether any of these would be legal on the continent I don't know but you would probably commit several offences simply by transporting them across borders and on board a commercial vessel if the item was classed as a weapon.
John and Judy I think you're taking a very sensible approach to manage a small but real risk. With basic planning and a few simple measures you can reduce the threat to yourselves considerably without the need to engage in a confrontation. Many of these individuals will not back down and with very little to lose unless an individual was confident he could face down a group of assailants it would be foolish to engage in any form of physical fight.
Whilst we'd all like to think we're capable of facing down this if the worst should happen the reality of dealing with a group of aggressors in a strange place means the outcome will seldom be as good as we would like to believe it will be.
I appreciate not everyone will agree with my views and that's their right but reality is tough to ignore.