You are correct about the tech industry here (low salaries relative to US) but developers working in finance in London make comparable money to Bay Area tech firms.

People routinely get paid £100k straight out of university to write code in London, just not in the tech scene. Total comp in the financial sector with 5 years experience is commonly £200k.

Which is not 'ludicrous' but more than you might imagine from seeing £40k job postings in the web dev sector.