Surely caring about what the neighbours think is the epitome of ‘non-U’?
I think it's more a matter of public/private divide stuff, like eating on the street, but -- as @LaMarschallin (who has a worryingly total recall of the entire works of Jilly Cooper
) says, those whose unimpeachably high social prestige is not ever in question, can rule-break at will.
When we were at university, DH used to bring home for dinner sometimes a minor royal he used to play intercollege football with, who had the most revolting table manners, wore shiny tracksuits, used to do heroin, not in a 'glamorous aristo' way, but in a 'with the addicts behind the bus station' way, and was planning on a fairly scuzzy post-university job.
They lost touch, but I noted more recently from the media that he's married someone very suitable, is now in exactly the kind of job you would expect from someone in his social bracket, and his children are called classic Aristo Child names.
Some rules you break, some you really don't, I suppose.