I am afraid you have a view of the U.K. which I do not recognise, you seem to believe that most of the criminals here carry guns too. Whilst I accept some do the vast majority do not, and the reason for this is armed robbery or any other crime is treated proportionately heavily when the criminal is caught and they know it.
I think that you are projecting American standards on the U.K. and it does not fit.
Pete from Peterborough UK
You do know I'm from the UK, right?
Regardless of what weapons a criminal carries, they are weapons that are denied to the law-abiding. A criminal carrying a knife bigger than the pathetic restrictions within the UK is dangerous, sufficiently so that the law-abiding may reasonably wish to protect themselves from such a person. A criminal breaking into your house wielding a screwdriver as a weapon is likely to be dangerous.
Armed robbery, when "armed" means with a firearm, is treated severely although from a legal perspective the "armed" appears far more concerning than the "robbery". Personally I want the freedom to protect myself from someone breaking into my home, regardless of whether and how they are armed.
It's also a problem when "if they are caught" is a major part of the focus. I can't say I'm familiar with the mindset of a burglar but I'd hazard a guess they typically don't wait until the police are nearby to commit their crimes. In the UK if you harm a burglar during the course of his crimes the chances are you'll face charges. Across much of the US if you shoot a burglar the chances are you'll be investigated but end up not charged with anything.
ETA: Regardless of the presence of weapons, what is a slightly built person supposed to do if confronted by a much larger aggressive individual? Even if the thug isn't armed in the sense of carrying a knife or a gun, the balance of power is overwhelmingly in their favour. Do you believe that the victim in this case has no entitlement to protect themselves from harm, and that their only legal option should be to hope that the police find their attacker and hope that the courts hand down an acceptable punishment?