It seems also that the airspace, both amount, layout and vertical constraints of it play a big part in the ability to do CDA's, somewhere like EGLL and please dont take this the wrong way are probably easier to achieve than at somewhere else. In some ways having a continuous flow of traffic resulting in near permanent speed control make the application of CDA's easier, a quiter unit probably makes it harder, a single or largely spaced group of inbounds being vectored downwind, seems madness to suddenly give an a/c speedcontrol to make it slow its rate of descent just in order to achieve a CDA, yet actually make the flight longer.