I did a trip recently on my own and did the same.. I went round the M25 to Brighton via the supplying BMW dealer and back home.. Stopped overnight in Brighton.
I total I did about 260 miles and average 31.9mpg down to Brighton and by the time I got home averaged 30.6. A few roadworks with avg speed cameras kept the cruise control set at 54-56mph. Otherwise I cruised under 85mph and made an effort to avoid hard acceleration. However I did make the odd 100mph blat and joined the M25 off the M23 quickly... then when near home on my own 'private stretch off road' had a quick 2 mile 'play' with a 911C4S upto and beyond 130/140 - it did not last long
and still achieved a good average mpg over the journey.
Normally average 23-26mpg.