Don't have the book at hand to check but I think the ICAO 'pilots discretion' phrase is exactly the same as the UK one.
So, as Scott alludes, it should be clear that an instruction without either 'when ready' in most of the world, or 'pilots discretion' in the USA means that we want you to do it now.
I don't think we should have to spoon feed pilots in these busy times, it wastes our time and yours. And time is becoming a rare commodity both on the flight deck and in the ATC facility.