That is probably true, but what university is going to pay a prof $1M USD? I think the issue remains valid. Also I doubt it's only UK salaries that are crappy these days.
Sure they can't afford $1M but in general you are lucky to get $110k+ in the UK even if you have 10 years behind you, even more so outside of London, in London the money doesn't go that far.
It's a no brainer, if you can get a visa and happy to move even if for a few years it can make a big financial life difference.