As said time and again any descent clearance cancels the previous one, I dont think anyone in the UK would disagree nowadays. It is the controllers resposibility to assess you rate of descent to ensure you comply with the agreement and dont end up in another sectors airspace. Eg the 75D MID restriction is to give one london sector a chance to get you below another but to do this a descent must be given thereby canceling the 75D restriction so now the clearance should go "descend FL150 lvl 50D mid, fly heading 150 be FL 200 or below 75D Mid" assuming you dont have to stopped off due other traffic. (Say that to all your inbounds during a busy period and you will find youself very short of time. (not an excuse))
Throwing lots of numbers like this at some pilots for who english is not their best language can lead to problems, the clearances such as 130 by tanet be 200 by ering are less confusing as less numbers.
The point being although we are all aware that previous clearance is canceled it really helps the sectors out when busy if you can still comply without them having to be repeated and if given an expect lvl xxx by xxx although its not a clearance its an important bit of info in planning your descent for us to ensure you stay safe and sound in the appropriate peoples airspace.
Come visit london centres and see how many level by restrictions keep you away from other sectors and their traffic.
It may not be the best system but its what we have to work with.