In theory you can tell us you have left the level as soon as you have left it. Some pilots say "leaving" as they readback the new cleared level but we tend to be careful about jumping in with descent for the a/c above.
If we use the mode C we have to wait until we see you are 400' out before we can reallocate that level. You will appreciate that at Heathrow in particular we are often strapped for levels and it saves time to ask you rather than watch your mode C.
We expect 650fpm plus or minus 150 whilst you are in the hold. Once you are below the transition altitude that is no longer mandatory, in order that you can comply with the CDA requirement.
Hope this helps
Point 4
