In London you are competing with cash flow oriented industries that will set expectations for the compensation mix, so you are in practice left with somewhat marginal people and not the best. This crowding out effect, compounded by the relatively small exits produced in the UK and an inferior work ethic compared to the US, explains why both founders and employees are less good on average than in some other startup hubs.
For the UK startup scene to really succeed we basically need to destroy the banks and agencies.