So, only criminals are allowed to defend themselves?

There's an old saying, "an armed society is a polite society" which is why I suspect the UK is no longer a polite (or pleasant) place to live, after all, successive governments have done their level best to disarm the majority of the populace to the extent that only the police, armed services and criminals have weapons, anyone else is liable to arrest or harassment for carrying the means to defend themselves.
Mail.

Police forces across Britain have been warned to beware of a flood of high-powered stun guns disguised as mobile phones which are being smuggled into the country.
Operated by pressing a button on the side of the device, the weapons can disable victims with a million-volt charge – many times more powerful than a police Taser.
Authorities fear that the stun guns – marketed as personal protection devices – are increasingly becoming the criminals’ weapon of choice, with at least 10,000 estimated to be in circulation on Britain’s streets.
They are also becoming a threat to prison guards. A Chinese-made Kelon stun gun – identical to a Sony Ericsson K95 mobile phone – was recently found at Low Newton jail, where the mass murderer Rosemary West is an inmate.
Although illegal in Britain, the stun guns can be bought via the internet for as little as £15.
The devices are marketed on websites as ‘non-lethal personal security weapons’ for defence against muggers. Claims of up five million volts are advertised for some models.
Because stun guns have very low amperage, serious or permanent injury is unlikely to result.
A security source said last night: ‘Police forces across the country have confiscated and destroyed an increasing number of these highly dangerous weapons.’

Whilst such a device might be dangerous to law enforcement in the hands of criminals, it strikes me as being odd that they think that they'd be dangerous in the hands of ordinary people, after all other than the odd idiot or kid who might think it's fun to tazer someone, the vast majority of us simply wouldn't use it unless we were attacked. Similarly with carrying a firearm, majority of us either wouldn't or would not use it in anything other than self defence.
Yet such is the fear of us that the powers that be have that they do not want law abiding people having the means to defend themselves, will arrest us often enough if we do defend ourselves and are constantly on the lookout to further restrict the right to defend ourselves in the probable fear that one day we'll simply decide to shoot them instead, after all, they are the architects of our misery.
Sooner or later though, this lack of trust will backfire, it already has to a certain extent in the popularity of political parties and their memberships and the people will wake from their slumber and decide to do something about it, after all, there's only so many of them aren't there?