Prices in London do seem to have crept up over the last 2-3 years with an incall rate of £200/hr now becoming quite common and some guys starting to ask £250/hr (or more)
The most popular site for advertising in the UK currently seems to be sleepyboy, with far fewer guys in the UK advertising on Rentmen
It’s difficult to tell if the guys advertising on Rentmen are asking for higher rates than those on other sites such as sleepyboy (given the increasing tendency for rates to be listed as “ask me”), however it is possible that the reduced competition on Rentmen and the fact that other sites such as sleepyboy are perhaps less well known to international travellers might result in guys on Rentmen thinking they can get away with charging more
For some reason Villa Giani has always charged significantly less than the average rate for London (it will have very different overheads and a different business model). Adding a tip might also be more appropriate which may reduce the difference in price.