Cities outside London I generally see more convergence of rates, with most escorts and agencies charging somewhere between £120-£250 (with exceptions, brothels are often cheaper) but inside London there seems to be a lot more variety and diversity of rates, everything from £50-£1000, so I've always found it harder to figure out, especially seeing as I only venture in to the capital now and then (though I'm hoping to more often).
I think as others have said though, if you have other income streams that amply cover your general living expenses, start with a higher end of your realistic rate range preference then adjust accordingly. Of course it also depends on how you plan to work; you could try just offering longer bookings at a relatively high rate to start, take a decent deposit and just book a hotel room when you get a snag. If you are going to just book a room and then play it on the day, you'd probably need to go to your lower end rate otherwise its a bit of a pot luck if you are going v.high.