I guess in the NE agencies are reasonable. In London, they seem to make up a price and double it. I guess the brokers, bankers and American tourists pay those rates. Some posters on here do too, but looking at the reviews, it must be less than 1 in 100. Why? I think because great girls like HazelK and Adele and a few dozen others are at reasonable rates for great looks and service.
Suggest you start by looking at the lists of escorts (Hungarian, Polish, British etc) perhaps sorted by rating. Perhaps mention a few that you like, or just book one - pretty hard to go wrong with a girl that 20 separate punters liked - though as always YMMV.