As an American who recently moved to London, I agree with this. The salaries here are shockingly lower, especially considering how expensive London cost of living is. I wound up just working remotely for a SF firm, which I realize is a luxury thanks to my US passport.
Out of curiosity what area are you working in and if you're willing to share, what sort of numbers are we talking about?
London salaries have always been rather good in my view (if we ignore the shocking cost of living here) and although the higher remittances tend to based on experience here vs having a good degree, I'm not sure for me at least I could make more money anywhere else outside of the arab countries.
I'm a backend web developer (Django, Postgres, Redis) and I get paid right around 100k USD (plus some extra money in bonus and pension contributions). My understanding is that this is actually on the low end for SF/Silicon Valley.
I've interviewed for some UK jobs, and the best I've been offered was 60,000 GBP (which equates to about 75k USD at current exchange rates). I routinely see job postings requiring quite a bit of experience and expertise, and offering salaries in the 40-50k GBP range.
Honestly, I don't understand it. It seems like London tech firms are competing in the same market (in some cases, they literally are satellite offices of the same companies) as the US firms, so how do they get away with offering so much less money?