I punt only in Central London, choose well reviewed WG (low risk), go for 1h or 90 mn sessions more or less on a monthly basis. Then, I accept to pay up to £150/h and equivalent for 90mn (incall).
In the past, I have paid up to £250 for 90 mn a few times but, although these were nice meetings, it's not really VFM (of course all is relative to your bank account).
I feel it's less expensive in London than in Paris or other continental countries where it's illegal. In Paris, without the equivalent of UKP and then more risk, a lovely SP with pictures etc. will easily be at €250 or €300 per hour.