Companies pay the prevailing wage in whatever labor market they participate in. The prevailing wage in the UK is lower than in Silicon Valley, but higher than in, say, Oklahoma City.
Not sure why people are still confused about this. But it seems many still don't understand it. The funniest thing is when people try to concoct some sort of fairness argument to explain this, or equally hilarious is when they try to come up with a fairness argument to condemn it.
Different markets have different prices due to differences in supply and demand in those markets. Want developer salaries to be much higher in the UK? Create an environment where a bunch of big tech companies all want to massively increase hiring there. Salaries will go up. Want developer salaries to be lower in Silicon Valley? Get most of the tech companies to leave. Salaries will go down.
One can say that ultimately the reason why Silicon Valley wages are so high is because there is a productive ecosystem that was so beneficial to the creation of tech companies that there was an explosion in demand for local tech labor. That ecosystem consists of easy access to credit and VC funding, existing networks of suppliers and vendors, existing mentorship networks for engineers and entrepreneurs, stable regulations and enforcement of contracts/property rights, together with sufficient local talent to pull off the difficult job of company creation.
London didn't have that to the same degree, and so has a lower demand for tech labor with correspondingly lower wages.