Same thing seems to be happening with 30 minute rates. Not so long ago, 3 months, £60 seemed to be standard for half an hour, but recently prices have gone up to £70 or £80, and I've noticed a few at £100! This is in Leeds, hardly central London.
I'll admit I have paid up to £80 for half an hour, but only if a) the girl is fit and I can be reasonably sure from reviews on UKP that I will get a good service and b) that I can cum twice. That way my cum:cash ratio still works out at £120 - with all the will in the world, there's no way I'm gonna pump my muck more than 3 times in an hour.