This really isn't true and I really wish this myth would stop. Sure, if you go into the financial sector or find a startup where they don't know their arse from their elbow about software workers, you can dupe them into giving you more money. But 55k in London is a good wage for a software developer and you're definitely not looking at that if you're mid-level and working for a regular company.
Source: I've lived and worked (and continue to do so) as a software developer, technical lead and CTO in London and have been in several different sectors.
Source: I've lived and worked (and continue to do so) as a software developer, technical lead and CTO in London and have been in several different sectors.