I think it also depends on the agency as when I worked for one they had me listed as working every single day when I only worked 2 or 3 days for them in the end over a two week period. My experience with an agency was dreadful to be honest - I was taken to see someone who had asked for a party girl once (I'm not a party girl and had told them this). Also out of the £140 charged for the first hour the agency took £100. Yep, I was left with £40 for an hour's work. Also, I can agree with the negatives listed on here - because I only worked for them for about a week I didn't have any professional photos so they used some they found on the internet that looked a bit like me and I was always late to bookings because of travel time.
I'd say that in London there are mainly two types of agencies: The ones that provide outcalls at all hours for between £90-£150ish an hour. In this case the escorts work an evening for the agency, they will be in the car with the driver for the whole evening (8pm-5am) and be driven to bookings when (if) they come in. That's the kind I worked for and it was awful - no shower between bookings or a chance to clean myself up
The second type seem to be the more expensive type where the escorts usually have an incall flat somewhere in Central London, I don't really have much experience with them as none of them ever got back to me
Anyway sorry, if you knew all that already just have been lurking for a while and came across the thread and thought it might be useful/relevant.