"As soon as you are cleared for the intrument app. it cancels any previous restriction unless otherwise specified"
You can find this quote in the FAAO 7110.65, i'm not sure if this rule is applied per the icao regulations since well i can't find a suitable copy to read without paying for it.
But overall this is how it works here in the US. At a busy terminal, We often will use the downwind to reduce guys to 4000 and 210kts. Once they hit base we will take them to 180kts. Following the app clearance the ATCO will instruct the aircraft to maintain a certain speed in between 160 to 180kts until the FAF, OM, or a point at about a 5 mile final. After the 5 mile point the pilot is in commend of the required speed for touchdown. To do so we take in to consideration the compression effect. A 747,767 or even 757 will run down to about 110 to 130kts short final therefore to maintain a wake turbulence separation you must plan accordingly with the next aircraft. Give a little more room. An Airbus will slow more then a boeing most of the time in my experience.
But a good hint, or this is how i think the faster i get my final so if i can keep the final at about 180kts.......well the closer my final will be to the airport.....i don't want a final passed 20nm cuz that means more work for me. And well the faster the final the quicker i get the A/C to the tower so less R/T issues.
But well i hope this helps out the questions a little.
Take care people.