It seems mainly driven by the funding of the company. A large bank or oil company in the city can easily afford to pay Devs a large salary which is less than their investment bankers which generally means London.

Others like Just Eat made a massive VC raise and had money to burn so why not offer 20% above everyone else and get the pick of the best? They were advertising for Team Leaders in Bristol for around £130K, way more than most people across the country could hope to get.

Of course, with the salary comes the burden of perhaps being part of a much larger team, which might mean more variation, things not scaling or feeling lost in the machine, not making a difference in the big picture.