Whatever you do in/outcall specify which services you are most after when you book. The profiles will, for example, say DFK but when you get down to it some girls don't do it.
FYI: there is a Diva recommendations string on the London pages to help you choose.
Oh, and it pays to shop around. The girls on Diva are usually on a dozen other sites as well, and SOMETIMES the prices and services will differ.