I used to to half that commute on the motorway, Honda civic 2.2 diesel...
A few observations.
80 mph cruise control, you spend most of your time slowing down and speeding up for those doing the speed limit... No go.
70mph you end up matching the traffic flow until the inside two lanes are blocked by slower vehicles. Which means changing speed again. No no.
The fastest mpg rout is to find a coach, they regularly do the full 70mph . Just latch onto the rear of the coach at a safe distance, you'll probably jump about 10mpg increase over the distance with a low loss of time
Next slowest is to tag onto a fast lorry (one that's overtaking other lorries) brilliant as long as the speed is high enough you can slot between lorries when they overtake.
That used to be the best most reliable option, with the lower speed also you could be nearer to gaining 15mpg over the trip.
The most efficient I found (from a base line of 50mpg)
Was to follow a car carrier fully loaded, the turbulence from them is huge so they literally open up a huge pocket of air for you to enjoy, that netted me +18 mpg.
Which in fact ended up being a better result than sitting behind a 53mph lorry...
Consistency is everything though.
The car is rated at 53mpg but the driving like a normal person in normal traffic you'd get 50mpg so that's my base