You make it sound like this was some sort of heroic effort. It is (probably well paid) developers doing their job. That's it. No need for a parade.

Do UK engineers command salaries anywhere near SV level though?

No, they don't on average. This has been discussed many times on HN. Apps like this are outsourced to UK offices to keep budgets low. I routinely get offers and recruiter calls from the UK, I usually don't respond to them because I know they can't match US salaries because none of them ever have for the comparable US job.

Then there's moving to England, and the EU-related red tape. I love visiting the UK, but I'd never want to live there. And I'm a total anglophile.

For the same price as a grad in the US, you can get an Impbridge post doctorate with 3 years post-doc experience. Get US investors, but hire engineers in UK.

I'd say it depends on who's paying. I've seen differences of close to £40K/year for my position.

London software engineers can command higher salaries than elsewhere in the UK.

I hear the salaries are less than in SF though. Cost of living (rent) may be a little lower to match.

Yes, for companies like Facebook and Google, or in finance (where there's also a bonus).