So, mystery solved!
I got a new chain today and as I lined it up with the old one to measure where to cut, I saw the old one was stretched in total for the length of a whole link!
It's only 7 months old.
Which doesn't mean anything, obviously, but by my reckoning that should equate to about 3500km, which still doesn't feel much for a chain. I was expecting it to last way longer, even using it through the winter (considering there is very little grit in central London, if anything this winter)