I think money is one aspect of it - it’s definitely the most obvious/tenable.
but there’s also the networking, the confidence to hold your head up as an equal at business meetings etc.
I remember comedian Luisa Omielan did a documentary on privilege. She went to oxbridge to try and understand why graduates did so well in their respective fields, if it was all down to money, intelligence. She went to a debating hall and had a bit of an “aha” moment when she saw pics of previous debates - they’d invited big names like the Dalia lama to listen to students debate. Her point was that, when important people listen to you and validate you and pay attention to what you say - of course you have the confidence to go out to the world and speak your mind and put yourself out there.
and that isn’t limited to the likes of the dalai lama and oxbridge students. If your parents/peers enjoy a stable, comfortable social class and they validate you and listen to you… then you will enter the world ready to come out on top. Confident to just air your views and advocate for yourself. You’ve been groomed for it from birth. That path has been navigated for you.
and being middle class in the U.K. will automatically mean you get “listened to” or treated better by wider society and the media. Look at the media treatment of say, Maddie compared to Ben Needham. Look at the likes of Jeremy Kyle panelists and how they were viewed - when Kate moss does drugs it’s glamorous. When Bojo has multiple kids by multiple mothers it’s acceptable. Open a daily mail article that demonises the working class on any given day, heck look at some of the tattle threads on unpopular opinions, demonising people who “make a profession out of claiming benefits” but nowhere near the same level of scrutiny or scorn for tax avoiding dentists.