In sporting slang from the century before, it used to mean a race/fight/contest that had been fixed or that was in some way unfair, a swindle.
* https://archive.org/details/slangdictionarye00hottuoft/page/...
As the article states, multi-culturalism just leads to less use, and I am guilty of changing my ways to stay away from beloved terms, for instance 'Barnet'.
A variant of the phrase 'nicely chipped Barnet' can be used to acknowledge someone having a fresh haircut. Delivered with a nod and a smile this is neither a gushing compliment (to make them feel awkward) or ignorance of their fancy new hairdo. There is an aspect of friendliness and humour to it. It just breaks the ice. Of course this is lost on anyone from outside the UK (even Americans - as you discovered - although Australians and New Zealanders get with it).
As for Received Pronunciation, and 'almost'. That 'almost RP' is the pronunciation you want. RP isn't what it was, maybe Radio 4 English Accent is where it is today. People from all walks of life are okay with this 'posh' but not too posh accent. It is respectful of the person you are speaking with in much the same way as written English is respectful of the reader's time if you use spelling, punctuation and some structure.
With 'posh English' it has to be with the words Orwell would use - short ones rather than long ones. Yet we have a few people in public life such as the current Prime Minister who do the exact opposite, to bamboozle people with vocabulary, making them sound 'public school posh'.
When I moved to the US, I discovered I used two bits of it really often. "Scooby", that you mentioned, and "Butchers". Butchers Hook -> Look, e.g. "If you can give me a couple of minutes, I'll take a butchers at it, see if I can see what's going on".
I had to break out of that habit fairly quickly, cut down on a bunch of confusion for folks. A coworker at the time half-jokingly commented that he didn't understand half the things I said, but always figured it out from context.