At present a clearance automatically cancels any level restriction already imposed. I am at present trying to push for a change in the UK that will require a specific instruction to cancel any previous instruction with regard to level restrictions-but it isn't easy getting anyone to listen.
Why would you want the UK to do it different from ICAO and file yet another difference ?? We have a busy piece of international airspace with international carriers, not to mention lots of pilots who are not necessarily from the UK but flying UK registered aircraft, using it. Having our 'rules' 180 degrees out from the international norm is crazy, and setting up someone for a nasty. We have far too many 'differences' filed as it is IMHO.
If you can't restate the restriction because you're too busy on the RT, then delay the instruction till you sure the aircraft will comply by default anyway. Or file an overload since you don't have enough RT time to manage the sector properly.
The UK changed some years ago to a clear and unambiguous position on restrictions vs. reclearances, which fitted in with ICAO, precisely to remove doubt. It's in the UK AIP in black and white, and has been published in various internal UK NATS documents as well as UK CAA ones. Some folks seem in denial (in the UK) or are not up to speed on the procedure. That's an issue for their competency checker methinks
Downunder a requirement or restriction once imposed requires compliance; unless it is specifically cancelled or amended; or a further restriction or requirement is imposed.
I think your AIP is open to interpretation. In isolation, the quoted piece doesn't read well. Is a 'requirement' the same as a clearance or reclearance ??