Flymeboy
The answer to why you have to be FL200 45 DME BNN is simply because that is the standing agreement between Daventry andTC MIdlands. Midlands will then descend you to FL150 lvl BNN 25 DME.
As to when you should descend, I would expect you to descend right away, as there is always a reason to do so(certainly in TC airspace), and if the descent is to be at your dicretion, I will tell you to descend when ready.
In this instance, the descent is required because "ac transferred from Daventry sector to TC Midlands at FL200 may infringe TC Cpt airspace for a short distance" "TC Cowly is to ensure that LTMA inbounds vacate FL200 before the northern edge of the WCO corridor"
So there you have it, clear as mud!! Certainly as far as TC airspace is concerned, if we instruct you to climb and descend, we would like it done straight away, though if in doubt ask!!