Even if gun carrying became legal and more common in the UK I doubt it would lead to the sort of problems we see in the US. There just isn't as much of a trigger-happy gun-totin' culture about here, so at least a part of the problem -- the ease of getting hold of a gun for people who really shouldn't have them despite good intentions -- I don't envisage happening here.
All the same, an increased proliferation of guns even in trained officers or ex-officers would make me feel more uncomfortable, rather than less. Terrorist attacks remain rare events, so most of the time it isn't really a threat to our safety. To be sure this is partly the case because of fine work by the security services, but nevertheless terrorism is not a constant, daily threat to most people's lives. Guns being present in the streets all the time runs a risk of accidents, on the other hand, or of even the trained people with guns making a snap wrong decision that such-and-such is a threat, even though he wasn't -- and, if you could see the guns, would for me act more as a potent reminder of the possible threat than security against it.
It should be discussed for sure but in my opinion it's better to keep armed police officers as a last resort rather than a first one.