The UK has a GDP of over $2 trillion a year. The Prime Minister's salary is £140,000 a year.
If the PM was paid in the same proportion as the head of Save the Children then they would be on £140 million a year.
The PM doesn't do very much though really...there are 430,000+ civil servants actually doing the work
The current incumbent is barely worth £140 a year...not that he needs a salary at all
And there are no shortage of people who want the job for some weird reason and I expect many could afford to do it for nothing given they'd don't really have to pay for anything while in office...
I expect the the list of qualified individuals who could run a major charity well and efficiently like STC is actually pretty short
A very good friend of mine is a qualified accountant and finance manager at a large charity (around £100m a year), she earns maybe 50-60k...overpaid given she has 20 years experience and is extremely good at her job (much in the charity sector)?