I think we need to inject a bit of common sense in our attitude to "clearance" versus "instruction". The division between the two has long since eroded, and is a fine distinction left over from a bygone age of procedural controller assisted by radar controller, in the UK at least.
Remember that we deal with aircrew from all over the world, and that some UK aircraft operate in ATC environments all over the world. Rather than refer to a parochial document----which would be completely valid----we should strive to accomodate all our customers by injecting clarity where there may be room for doubt or misinterpretation.
Monarch A330: you may be working a high level sector to protect a low level sector workload; many Centres use a system of vertical or layer sectorisation. We don't always know your TOD, you may be a flutter-down feather type [the BUS] or a Boeing/exec jet dive bomber.....unfortunately some of our procedures are a one-size-fits-all......but those procedures move an incredible amount of traffic almost beyond the dreams of the initial system designers.
Just Visiting: you have identified the operators who make these requests. You could adapt your method of control to accomodate them and pre-plan your technique to suit. These people pay our wages. On the other hand it is unreasonable for an a pilot to expect a large section of airspace to be..in effect..reserved, if it is a busy environment. Alternatives are to issue a very minor level change and wait for commencement of descent......or just refuse the request until the aircraft is actually ready to commence....if there is a genuine traffic reason.