I found 60 mph on my Mk 1's speedometer gives me my best motorway mpg. It equates to 53-55 mph on the sat nav GPS. Mind you, that is without a top box. A top box decimates mpg, especially as speeds go up. With a car as heavily laden as yours the tyre pressures should be increased, when cold, to at least the maximum indicated on the B-Pillar sticker. Mine states 33 psi front and rear, for a loaded car, but I run 35, two up, and would go to the towing figure of 38 on the rear, if heavily laden.
80 mph, fully loaded and with a top box, would probably give me the numbers you are seeing. As John says, potter along with the lorries. I would sit in behind one (not too close as to be unsafe), and take advantage of them pushing the air aside for you. I have seen a 50% improvement in instantaneous mpg doing that.