I can shed some light on the M135i but Ketron can give better feedback of long motorway journeys.

I've a 35km commute, 70km daily, which consists of 25% Motorway at 120kmph, 25% National Roads at 100kmph and 50% B Roads at 80kmph and at 6am if I'm tipping along, so no real traffic to speak of, Im seeing 35+mpg. Then maybe about 33mpg in the evenings with a bit of traffic. 30+ is easy even with frequent squeezes of the throttle leaving junctions and merging onto motorways. On days off, around a busy town, you'd be down around 22-24mpg.

The only downside to the M135i with long journeys is the tiny tank. Other than that, if you will be leaving it sitting on the driveway a day or two of the work week, then I'd say it could be considered!