I'm from the UK and moved to the US 10 years ago.
Software engineer salaries in the UK are on the 75th percentile of earnings vs the 90th percentile in the US. The US also has higher variance of income, so at the 90th percentile one would earn 2.6x the median salary vs 2.0x in the UK. If you are a highly paid professional the US is a pretty good place to be.
Just before the banking crash of 2008 pay for software engineers in London working in finance were fairly competitive with Silicon Valley. You generally got paid better as a contractor, £500-600/day which was then about $950-1150/day which if you worked all year (six month contracts were common) worked out at £120k/year ($230k) which was about double what you'd get as an employee so lots of senior engineers would do that (no need to worry about healthcare!)
Since the crash of 2008 the US dollar has increased a lot against other currencies and has grown much more than Europe. The Obama administrations stimulus helped the US recover more quickly while the Eurozone was forced into austerity while it was forced to work through the contradictions in the way the Euro was set up, with countries issuing debt in a currency they no longer controlled.
The US tech sector went through a huge boom in the past decade which drove up tech salaries here more generally. It was also pretty good for UK tech salaries as Google and Facebook opened large offices in London.
The UK was a huge financial centre similar in size to New York with bank bailouts spread over just 60M instead of 300M people, but being outside the Euro the UK was not forced into austerity in the same way many EuroZone countries were. The Tory government used it as a way to shrink the state. This stymied growth and ultimately sowed the seeds for Brexit which I think was a misguided anti-establishment backlash, in its way quite similar to Trump in the US but with longer lasting consequences.
The government's Office of Budget Responsibility estimates Brexit to ultimately end up costing about 4% of GDP over 15 years. Certain financial services jobs are moving to the EU for regulatory reasons and it could well kill off much of the UK car industry which was heavily integrated with the EU. The UK was particularly focussed in engines/powertrains so will be heavily hit by the switch to electric vehicles and there seems little reason for global car makers to invest in the UK when they could invest in the much larger EU.
Even now, software developer salaries in London are higher than pretty much everywhere else in Europe except Zurich. But the rental costs are just obscene. Sometimes I think about moving back to Europe for a while but I'm not sure I'd move back to the UK since after a decade of austerity it all seems a bit grim.
Even though we will get rid of this awful Tory government soon the Labour Party has promised not to make any substantial changes to ameliorate the costs of Brexit, ruling out rejoining the single market.