Outside of London’s Metropolitan Police, most British cops have sidearms. The last time I was in Gatwick, they had MP-5s.
The vast majority of British forces still don't routinely arm.
Airport police are an exception, as are the Civil Nuclear Constabulary, MoD Police, but the only provincial force which routinely arms its officers is Northern Ireland.
Only AFOs (Authorised Firearms Officers) are allowed to carry firearms, and in no force are more than 25% of officers AFO qualified. There are also SFOs (Specialist Firearms Officers) but they are even rarer.
You are more likely to see an armed officer out of London, simply because they authorise a much higher number of officers as AFOs (in London, it is felt that Armed Response Vehicles can respond quickly enough and so only about 7% of officers are AFOs - in a county force, servicing a much wider area the situation is different) but they are still the decisive majority.