This is a job for a charity - you're never getting stock + bonuses + competitive pay in a third sector job. UK pay is not near US but this is probably still median SWE pay outside tech roles and London + FAANG + others will pay closer with what you're suggesting with the mentioned bonuses/stock.

Stock... in Stonehenge?

I think comparing a job like this purely on salary terms misses a lot. It's a prestige job that will be the highlight of someone's CV for the rest of their career. Not to mention 25-28 days vacation.

As someone that's lived both in the US and outside of it there's no denying US salaries are top of the game. But there are a lot of other factors that go into a person's life than salary alone. Long hours in US jobs are not rare at all. I expect folks at Stonehenge are out the door at 5pm sharp.