At the very least you would know if you are doing something that would merit being arrested.
A random person approaching you and trying to arrest you while you are not committing any offence is not a common occurrence at all I would think
It's not like one would be arrested often or at risk to be arrested by a fake officer often?
I live in Brixton, not a million miles away from where Sarah was kidnapped. There is a mugger/crack addict who has worked Coldharbour Lane for at least the last five years, with an apparently genuine warrant card that I assume he’s nicked from somewhere. Plain clothes, says he’s undercover. He has approached both me and DH, separately (threatened to “fit DH up for rape” if he didn’t give him money, suggested he’d arrest me for being drunk and disorderly unless I gave him a blow job). Both times he tried to get us to leave the main road “back to his police car”. Early evening both times - I was coming back from work at about 8pm. We’ve both seen him multiple times, and know to avoid him now. Various friends and neighbours know him too. One of my colleagues lost his wallet to him - he took it to “check his ID” and he ran off with it. He isn’t particularly convincing - he moves to demanding money/sex too quickly to be believable, and is clearly trying to get you to go with him. But no, it isn’t unusual. We’ve both reported him, the actual Met aren’t interested because apparently no crime has been committed.
I was on the train between Woolwich and Denmark Hill about two months ago, and a guy there also claimed to be an undercover cop. Had an obviously fake warrant card, and a non-functioning walkie-talkie that he was talking into. He tried to get a woman to leave the train with him at Blackheath, which is a very quiet station outside of rush hour. She kicked up a fuss, refused to get off, and he legged it at Lewisham. Again, BTS weren’t interested “because he’s gone now”. This was after Sarah Everard. Obvious copycat. The police didn’t give a shit.