I think that Finncapt strikes at the core of the problem - de-icing facilities.
If you look at the way 'good' airports (OSL, HEL, MUC etc) manage winter weather, it is almost wiithout exception that they have a de-icing area, usually near the end of the departure runway so that you taxi out, get de-iced and take off with little delay. It makes for better recycling of the fluids and the operators are very proficient at their job.
Where you would construct such an area (or 3, 09R, 27L, 27R) at Heathrow?