Flanker: Thanks for your input. I do understand what you say, but I think it is important that you understand the relevance what you are doing has in the wider ATC perspective.
I would divide the three methods of achieving descent from an ATC position as follows:
1) Descend when ready: You programme the FMS and let it follow the profile it wishes. This is the system you are describing.
2) Descend now: We need you to start a descent perhaps before the ideal FMS profile to solve conflicts with other aircraft.
3) A/c requests descent: From an ATC point of view, if YOU ask for descent, I expect you to start descent IMMEDIATELY upon receiving the clearance. In most peoples' mind the act of requesting descent means you want down now, not in 15 miles' time. As I have said, by giving you clearance the controller might have formulated a plan to deconflict you with other traffic, and if you don't descend you muck up that plan.
Some airlines are starting to make things a bit clearer by saying 'Requesting descent in 15 miles' or some such. That at least gives us an idea of what you are planning to do.
If you want a short advice, I would say if, in UK airspace, your MCP panel prompts you to set a new cleared level and you request and receive descent clearance, then start down upon receipt, if necessary using the procedure already outlined above with reference to Britannia. Then at least we are all using the same plan.