Our police do have guns here now though, or can carry tasers.
The issue is escalation. At the moment, people who break into your house in the UK aren't likely to be carrying a gun, but if everyone had easier access to guns, it follows that they would. On the plus side, you would have more power to defend yourself as a civvie, but the criminals would cotton on and start carrying equally offensive weapons, because they probably aren't going to stop stealing or doing what they do. On the whole, it would make the country a more 'dangerous' place. And there are other things influenced by guns, like:
Some research shows an association between household firearm ownership and gun
suicide rates.
[10][11] For example, it was found that individuals in a firearm owning home are close to
five times more likely to commit suicide than those individuals who do not own firearms.
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Kellermann, A.L., F.P. Rivara, G. Somes, et al. (1992). "Suicide in the home in relation to gun ownership". New England Journal of Medicine 327 (7): pp. 467–472.
The U.S. still has a high crime rate on average for a developed country out of the list of world countries, even though citizens can defend themselves and carry guns, and yet there are countries where citizens can't and the crime rate is low. Therefore I don't think crime rate is related to being able to defend yourself with lethal force, it's related to social attitudes and levels of civility. Japan has a very low crime rate per capita, including gun crime rate, where gun control is very high. As do some European countries where guns are not easy to get hold of. It's probably more to do with the public attitude and relationships with the police rather than whether or not they carry guns, or someone would take advantage of that. Here, we tend to associate the ownership of guns with two things: farmers and sports shooters. We don't tend to expect our friends to just have one in the house 'for protection', as we don't see ourselves as needing that protection. But things may be changing.
You could argue that if Norwegian kids were able to carry firearms legally for defence, then Breivik may have been stopped before he killed scores of them on Utoeya. But then the chance of another Utoeya is also slim because guns are not an accepted part of Norwegian culture in the way they are in America. You run the risk of vulnerability if a person takes advantage with firearms in such a situation, but on the whole you are less likely to even see an occurrence like that in a nation that doesn't back personal gun ownership to the extent the US does. So which is best? I still think that a non-gun carrying nation with associated attitudes is generally safer than one where everyone carries guns, for that reason. Where everyone has guns, you are likely to see more gun crime, more gun-related homicides, and more mass shootings, simply because they are there and the temptation is there for some people to abuse them.