In the UK to own a gun you need to have a good reason to do so. This is usually by being a member of a gun club, and getting a license to own a gun. Once you have a club membership and a license you then need to start speaking to the police to purchase a gun alongside having a gun box that is secure and certified by the police. Its an awful lot of hassle so the only people with guns are those who are interested in it.

Police dont tend to have guns apart from specially trained units(?) so you only see these quite rarely. Northern Ireland is a bit of an exception all police in NI have a gun, and are trained to quite a high standard to just be able to use it. NI's police actually go over to England reguarly enough to help when police with guns are required.