Just my tuppenceworth:
"Descend" means NOW;
"Descend when ready" means just that!
It is also worth reiterating an early point which got swallowed up:
If you ask for descent and are given it, then DESCEND. Don't sit there for another 10 miles waiting for the FMS to reach TOD and then wander down. In many of the busy sectors such as Clacton, your request for descent will mean that the ATCO will make a plan for you and the other aircraft based upon you descending when you ask for it, maybe ducking under another aircraft which was borderline for descent before you but now will be subject to your descent. If you mean "We would like descent clearance to start down in 15 miles" then make that clear. You are pilots, not computer operators, after all!! Fly the aircraft, not the computer.
With regard to the "Can we descend at 500 FPM if you give us early descent until we reach the profile" question: Well the minimum ROC/ROD in the UK AIP is 500 FPM so of course the answer is YES. Bear in mind, however, that if you are asked to descend early it is probably because we want you down so that we can also start descending other aircraft who need to be at the same level at the same point as you. If you all did it on the ideal profile then you would all reach the same point at the same time at the same level, and this is generally considered to be a BAD idea in ATC circles! Look at it another way: If you are given descent early, it is likely that you are being put ahead of other traffic, so it's GOOD thing!