I'm not from London, not have a used mobike, but one of the criticisms of obike (one of the schemes started here in Melbourne) is that they've apparently dumped a supply of bikes that SEEM like they were designed for a population of much shorter people (the assumption is east Asian populations).

Either way, when half the male population is 6ft or above, it seems the bikes are just generally too small, even with the seat extended to their maximum height.

You can a bit, but the max height is still absurdly low. Where Boris bikes will probably allow a 190cm tall person to be comfortable, these probably max out around 170cm tall before you're feeling the pain.

I'm 202cm and they're fine for me. Was amazed the first time I tried

I'm 194cm and for me it was a bit too small even when I adjust the seat. I can still ride the bicycle fairly comfortable but I need to be careful as I sometimes scratch my knees against the handlebar. I had few scratches on my right knee last week from riding Mobike so I had to adjust my technique a bit which means I cannot go as fast as I could and can't turn too rapidly left or right.