It rarely happens. A beat officer is unlikely to ever find themselves facing a suspect armed with a firearm in the UK. Most gun crime in the UK is gang-on-gang, they seldom use guns against the police. Which isn't to say that it never happens, but when it does it's noteworthy simply because of its rarity. The other times you get armed suspect will be hostage type situations, at which point armed officers automatically get deployed anyway.
The only place you'll find routinely armed police officers in the UK are at airports, MOD plods (civilian police responsible for policing MOD property), and the CNC (Civil Nuclear Constabulary - responsible for policing nuclear establishments in the UK).