I must live an economy lifestyle to put anything aside - taking care to not heat my home too much, to never eat out, and not buy any luxuries. And on some months when I need private healthcare (as the NHS has collapsed to the point that some urgent appointments have months-long waiting lists) or dental costs, I have to dip into those savings.
I'm leaving the UK as soon as I have savings to do so. But it might take a while. I feel a lot of empathy for people earning around the national average salary. I don't know how they'll survive the Winter without using savings or going into considerable debt, especially those who have dependents. I am privileged to at least not go into debt and realistically think about leaving this country.
Regarding the London fintech experience, you are right, it is comparable to Mountain View. The financial industry in London is a well known exception to the rule when it comes to salaries in the UK.
I'll use Discover Card as they are a financial services company in both the US and UK.
According to Glass Door, a Senior Software Engineer with 15 years+ earns:
UK: £90-100k
USA: $120-140k
Even at the top end of finance, with the most experience you will struggle to break the £100k wall in the UK.
As someone who has worked software in both the US and UK, you are more likely to find upper management in the US who can see their software as a competitive advantage.
In the UK I've seen upper management in fintech startups embrace the advantages, but not in larger organisations.
That 150k, assuming pounds, is a bit under five times the normal median income. Assuming dollars, the same number would be true for the US.
UK wages aren't low. You're rich.