I usually fill up with BP normal (95) unleaded, and have had a maximum of 26mpg on my regular run north @ 75mph (average speed showing 69mph after 185 miles). 2002 S8.

This time (thursday night) I filled up with BP Ultimate, grumbling as I did so at 100.9p per litre but...

27.9mpg @ varying speeds (bank hol traffic) average speed showed 66mph after 205 miles. I'm pretty optimistic I may be able to break 28mpg going back south if the M1/A43/M40 are quiet enough (from Barnsley to Chalfont St.Giles).

This isn't a big-up for BP, just my experience changing my usual fill, as I tend to use the same BP garage when south. I'll probably fill up with something else 97-99ron when I leave Barnsley, see what I get.