Taking just one level away from us (it is mandatory, not to be done at our discretion during quieter times), means that we will have to hold out sooner. (Standing agreements start as low as FL130, giving only 4/5 useable levels at the second busiest EGLL stack)
Holding out causes en-route a lot of problems, the hold areas are huge and the time to take aircraft off and bring them on is big.
Any extra fuel saved will be lost in this process - that's not-withstanding the very pertinent points Del Prado has made.
To keep runway occupancy, you need to have aircraft at the lowest levels in the hold... I can release them to EGLL at FL90, but the first thing they will do is drop it to Min Stack, or min stack+1 depending which hold it is.
Higher level holding is a good idea if the runway is blocked or unavailable - not so good when you are trying to keep occupancy levels up.
You can bang on about fuel burn all you like - the theory is correct, but the putting it into practice is complete fantasy when you are trying to land aircraft!
To make the practice mandatory regardless of traffic scenarios or loading is insane. The GS who suggested this (you know who you are, stack swap king), has just confirmed the belief by many in the ops room that he is clueless about TMA (Area) operations... Of course it's been siezed upon by office workers who see anything with the 'green' label as being good - without thinking of the ramifications.