I've lived in several European capitals (and in a major US city), and I've never the same "oh you live in flat, you must be poor" reaction.
Especially when you consider that flats in some parts of London are bloody expensive too!
One of the perks of living in England as a foreigner is you avoid the extreme class stratification that is English society or are at least given a pass.
And yes, to certain circles saying you live in a flat is akin to admitting you're poor. This along with your accent also identifies your class.
I avoided this by calling my flat an apartment though I lived in New Cross near Deptford which is definitely "low class" or as I liked to say, " gentrifying.".
Funny on apartment / flat. Why is one better than the other? In my mind there's no difference. Is it just some sort of subconscious connotation?
With that said, I loved england and all of its quirks.
The majority (loosely/anecdotally speaking) of the particularly desirably located houses are converted to flats.
I know that, but that's not what most londoners believe.
Native here. Born '74, perhaps ironically I lived in a house when young because my family were poor and we lived in charity housing. In '99 I moved out and lived only in flats until eventually leaving London in 2021, moving back into a house at age 46. Suffice to say I don't recognise this attitude in myself, nor any of my friends or colleagues throughout those 46 years. TBH I knew very few people that lived in a house in London for most of that time anyway.