According to multimap: 139.78 miles

140 miles / 110 minutes = 76mph average

Not bad going considering theres about 8 roundabouts just to get back to the first motorway section and thats literally door to door you averaged that speed including the time taken to walk to and from the car etc.

Speedo must have been WELL into 3 figures almost the entire way home to manage that sort of door to door average