Ive just come back from Wales, so M4 all the way., I averaged  37.1 mpg. Drove 50 miles and had a glance at range remaining, and it said i had 380 mile range available still!! 

.Did Bridge End to Southampton, on just over a quarter of a tank  

. Boring trip mind you sat on cruise control 

Pottling around wales, because of its lack of traffic lights and traffic cues, i was averaging 32 mpg, compared to my normal 26mpg around Southampton.
So goes to show it all depends on where you live as to what Econamy your likely to see.