"a Nothing transmission"??
It amplifies the Approach plates, which state standard speeds and when they are to be achieved by.
Regardless of nationality, if the pilot can't interpret the approach plates, he should not be flying.
BTW I am NOT a Heathrow controller, but was using thier hold for an example, the same is true for any other hold at any airport with instrument approach procedures.
Giving a specific speed in en route controlling is, I agree, the way it should be done, however it works slightly differently if you are not using speed for separation, which is the gist of this thread.
The phrase is only usually used if questioned by the pilots, which they should not really be doing anyways as they should follow standard procedures unless instructed otherwise.
These standard procedures include standard routes, standard speeds, standard rates of descent, standard holds etc.
Short of flying the aircraft for them what are we supposed to do?? There are 2 of them to interpret the charts, the workload, despite any pilots protestation otherwise, is not so huge as to make this an impossible task.
The only time (in my opinion; having flown professionally for 12 years), that the wokload is especially 'high' is in an emergency; and in that circumstance i would offer every assistance, including giving specific speeds etc. Even then unless it is a catastrophic failure such as total engine or decompression, the workload is not that high.