Yes the limited data on this web site for escorts with 3+ positive ratings backs this up. (Who cares what the poor providers charge?) We only have good data for London, EMids, WMids and SE. London is the same cost as the Midlands. The SE is more expensive. Brits in London cost more.
(You stat experts - I admit we might get a different conclusion comparing Mean, Median, and Mode, but that is taking it too seriously)
I think it is because even £100/hour is loads of money. So they can afford London costs. And so the big supply in London keeps rates under control.