I drive 100 miles a day on motorways and I think 80 should not be a problem, stats suggest more than 50% do that anyway and plod do turn a blind eye. The problem is when plod are driving down the motorway people brake to 70, in heavy conditions this actually causes traffic to come to a grinding halt - very quickly. Allowing 80 then maybe this will be eased.
Jake has a point, at 50-60 I can get 28 mpg (diesel) at 70-80 it is around 23 mpg and if faster can drop to as low as 17mpg.