Detectives and other “senior” police officers in UK generally work in plain clothes. They aren’t generally on the beat shaking people down, but are otherwise regular police officers, can and will arrest you if need be. But then this is UK, citizen arrests are rare but also 100% legal in appropriate circumstances so it’s not a biggie here.

The law says, IIRC that plainclothes officers must show their ID, without even being asked for it, when approaching people.

So it’s not like they are secret spooks.