If anything the contrast is even more stark across the EU
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.
Well, yes, but the topic under discussion is whether the market is "developers that can commute to the local office" or "developers they can work on that timezone" or "developers anywhere," which all have different implications for the market clearing rate.
This probably isn't true. A quick googling indicates the average software engineer salary for Oklahoma City as being in the 70-80k USD range, while a similar googling says the number for the UK is around 50k GBP, which is...68k USD.
The median software engineer salary for the US as a whole is 110k USD according to the BLS, so even cheaper areas like Oklahoma City won't be super low.
Post tax, I'd expect the US advantage to increase in raw monetary value, and the cost of living in Oklahoma City is probably very low for a sizable city, but of course the UK would offer superior benefits.
Qualified doesn't mean you can churn out so many lines of code or have X years of experience at a company or whatever, it means you have right kind of experience. To a big company like Facebook or Google, someone from SV who has a few years in the local industry has more relevant "experience" to the position than someone in London with 10 years at a financial firm.