The teenage use most people are talking about is quite distinct from the original usage, I believe. I think the OP is referring to the latter (since she isn’t hearing London teenagers come out with it at all).
The original was very much twee and ‘oh I mustn’t say oh my god because it’s offensive’.
It is kind of annoying when teenagers take an existing phrase and change it, and then act as if the original never existed. Inexplicably they’ve also done this with ‘low-key’; this obviously means ‘mild’ or ‘unobtrusive’ but for them it seems to mean ‘genuinely’ or something 🤷♀️🤷♀️