Armed in the sense of with a firearm, tazer or similar? Yeah, you're right, only a minority of British cops are qualified for that and most wouldn't have them anyway unless they need them.
But keep in mind in the UK all weapons are prohibited. Walking down the street waving a baseball bat in a threatening manner is a criminal offence in the UK. Any object that could reasonably be interpreted as a weapon needs a lawful excuse to carry it in public, and a bunch of objects that violent people tend to fixate on are just outright banned, there aren't any excuses permitted for those, you just can't have them in public at all.
Thus having just the _same_ gear as the cop: pepper spray, an extensible baton and some hand cuffs, would get you in trouble here, never mind any sort of firearm.
"Constable" is the legal term used in UK law to refer to police officers and give them special powers or immunities. A Detective Chief Inspector is still a Constable by this meaning, so is a Special Constable (volunteer spare time cop) but a Police Community Support Officer (cheaper fake cops with less training) is not a constable. Outside of statute law, ordinary people don't call them constables.
British pop culture says the equivalent of a "get out of jail free" card would be membership of a Masonic Lodge. Police officers (in particular senior ranks) are supposed to be particularly likely to join the Masons, an esoteric order with a big emphasis on helping other members.