The last southbound overnight Megabus I took did Glasgow Buchanan St to London Green line Bus station in 7 hrs 2 mins including about 10 mins for driver changeover at Forton Services, about 20 miles of 50 mph average speed cameras have gone on the M6 since then. Knock off 15 mins starting from Euston vice Victoria and even Edinburgh wouldn't be that far fetched. We didn't use the M6 toll not sure how much time that saves.