i have a 475 approx run coming up X2

east midlands to past inverness, then back a week later. aa route finder recon about 9hrs +

I have no particular schedule to keep or times that i have to arrive at either end - but i want to do each leg in a day.

whats the ideal best speed to sit at to get best fuel economy - most of route will be M6 motorway. i am happy to roll along and enjoy the scenery going past
does cruise control get better economy than a careful right foot?