In the UK, the criminals ~never have a gun (and less crime occurs), so it makes sense that the police wouldn't need guns either.
In the US, normal citizens are often legally armed, and criminals are very often illegally armed.
It would make no sense to tell an officer "you can't carry a gun" when both average citizens and the criminals have guns. While the UK undoubtedly has a fine model for itself, it can't be extrapolated to a country where gun ownership is so common.