HIGH 30s? that seems optimistic, does your commute involve a long downhill section?!?
my commute is also 90% motorway, 200 miles a week. i'm normally cruising at 75-80 (same as everyone else on the M40) and average 28mpg.
My commute involves sitting in the passenger seat of a 1 series, so I get mid to high millions of MPG
35, 36, 37, 38 is very possible on motorways if you don't have to slow down and speed up again.