The issue is not de-icing but de-snowing. BA and other’s processes work well for de-frosting but add an inch or six of snow and the volume of de-icing fluid needed goes up 40 fold.
Try ‘de-snowing’ your car with de-icing spray and see how many aerosols you need…..
In addition, heavy snow falls in the middle of the day at LHR (when most of the long haul fleet are on the ground) has always created chaos in my 40 yrs experience. Not only do the airline need to de-snow many widebodies but the BAA need to clear the stands (almost impossible when occupied by aircraft).