Having worked in a "posh place" I think this is more a question of nationality. We used to get lots of wealthy Merkins, they didn't hesitate to complain!! Not that there was ever much to complain about.
But if this was me I'd go quietly potty. You do not pay good money for sh!t food and service. Why would you?
sounds like there was actually nothing worth the money there, unless the wine was OK.
You do have to complain on the spot. But you can just quietly call the waiter/ess over, ask to speak to the manager and get him/her to say if they think it is worth the money. Make them taste if off your plate.
Eton mess is not supposed to be sour at all.
Plenty of places ont web you can review it and also say if you got redress or not.
Being posh means nothing, any eatery should give value for money.
Sorry mate, hate to hear about this sort of thing!