Got to agree with Scott 100%.
ICAO, and even the US
has a procedure which everyone wherever they fly in the world knows to be the same. The UK just finally caught up a couple of years ago.
It is crazy that with our level of international traffic in the UK we are talking about doing something 180 degrees out from everyone else.
Instead of changing the book, if we really can't cope with the traffic levels, then
get those sector capacities changed downwards. Don't introduce a procedure which will serve to cause confusion and the possibility of pilots reverting to the different system used worldwide in error.
Passing the restriction with an amended clearance is fail safe. Assuming the pilot is still going to comply if it is not stated isn't.