There is going to be a tipping point if London property prices continue to rise. There's no point in having a start up in, say, Shoreditch if you can't afford to pay living salaries to people to work there, so either you a) move somewhere the salaries and office space is cheaper or b) you go remote and increase your talent pool. There's quite a few startups that have to be in London (e.g. they work in the finance industry) but there's likely more than a few that really don't need to be.