Sorry to say I think you are mostly missing the point. Or perhaps I am.
I read the article to mean the powers to be have decided that class D should be readily available, and that lack of staff should not be the reason for restricting traffic.
Perhaps this is an attempt to ensure privatisation has not reduced staff to the sort of level that happened in Zurich.
I am concious of the workloads experienced, and on the occasions when I have thought it safer to request transit of LHR or CITY, my request has always been granted. Appreciated and Thanks.
Dutch
You are correct with respect to time, but not with respect to safety.
The NW corner of London CTR, and the easterly route DET LAM are very busy areas, as is the corridor BPK to North.
While the London ATC units usually give a pretty good FIS and RIS, Luton and Stanstead do not appear to have the same capability, outside the zone, so I can understand why they don't want you inside their zones. Perhaps they are not staffed to an appropriate level.