It seems a waste to write a long answer on Quora and not post it here too. So, here’s my answer to “What do British people mean when they say, in a derogatory(?) manner, ‘oh, this is so middle class!’?”
It’s interesting to read the answers here because so many of them indicate the class of the person writing them, whether that’s acknowledged or not.
For example, Alec Fane says:
The middle classes however… They try. They try to be posh, and proper, and correct, and jolly good fellows. But most of all, they try to be seen as these things.
Some of them do. Others have a kind of guilt about being middle class and would actually prefer to be seen as being more working class because that gives the impression of being more down-to-earth and “authentic”. One test would be to see how a middle class person behaves when talking to someone who is more working class – maybe a plumber or builder they’ve got round to do some work. I bet a sizeable proportion (including, I admit, me) will try and fit in by talking in a way they wouldn’t usually. It’s also possible this is more of a male middle class thing.
And Tom Alexander, who admits to coming from an upper class background, says:
That type of phrase is basically an example of elitism, where the person is basically saying “oh, this is so common!” in a disparaging manner
I’m sure the phrase can be used in that way, but no one I know (as a middle class person with generally middle class friends) would do so.
So, aside from the point of view of the people writing these answers, you have to bear in mind that there’s no single middle class. To say that a third (or whatever) of the population thinks the same way about something as complex as class, and their place in it, would, of course be over-simplifying.
To talk in terms of newspapers, a middle class person who reads the Times will think differently to one who reads the Daily Mail, who will think very differently to one who reads the Guardian. Partly this is political differences, but it’s not just that.
(For those less familiar… the Times is a right-leaning “quality” newspaper that (in my biased opinion) is less quality/intellectual/worthy than its readers like to think. The Daily Mail is a right-wing tabloid with sensationalist, alarmist headlines that, at best, bend the truth. And the Guardian is a left-leaning “quality” newspaper whose readers are seen as soft, hand-wringing, liberal (in the American sense) intellectuals who don’t live in the real world. (That’s me.))
A (stereotypical) Guardian reader might say something is “so middle class” in a self-deprecating way – “We’ve got this brilliant recipe for spelt loaves for our bread maker… oh god, that sounds so middle class!”
Whereas I can imagine someone who might objectively/demographically be identified as middle class, but is much more “aspirant” and sees themselves as somehow better, might say something is “so middle class” to distance themselves from what they see as a more common behaviour (like Tom Alexander, above).
And someone who is demographically middle class (in terms of income, home, job, etc) but who thinks of themselves as more working class (perhaps because their parents were working class, or because of where they grew up), would say something is “so middle class” to mock a pretentious, trying-too-hard behaviour that they see as ridiculous (whether it’s actually something they’d also be close to doing themselves or not).
As with anything about class, this is a far from clear answer. The short answer to your question is “it depends”!