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.