I was once in a car that was pulled over at a speed trap in northern England, and noticed that they had guns on their hips. When asked, the officer said that they manned low-priority things like speed traps the majority of the time that they were on duty so they could rapidly respond to any firearms-related incidents. With how few of those we typically have, it sounds like an incredibly boring job.

They have them sometimes in central London, I suspect when there are specific events/threat levels are higher.

I've seen them a few times when I've been walking round London, always a bit shocked when I stumble across them.

You'll often see armed police outside court houses for high profile or gang related cases in the UK