Indedd tbaylx. When this topic first came up, I made reference to the Holds in the LTMA. If the UK govt wanted to cut down on emissions, they only have to get the CAA to limit holding times. That means cutting the number of flights into EGLL and so nothing will happen. The gas used in the Holds is staggering. You only have to look at the number of machines per day and the average amount of time that they spend holding. The computation would then be easy.
Your info about spacing over the sub-continent is most interesting.