According to a survey by Santander reported in the Daily Mail, the average first-time house buyer has spent an average of £42,000 in rent over the previous seven years (double that for a couple).
Over a 25-year term it reckons renters will pay about £300,000 while home-owners with a typical 75% mortgage on a £150,000 home will pay £212,000. All these figures would be significantly higher in London.