The RAC: 272.5 miles, 6 hours 36
The AA: 297.1 miles, 5 hours 57
MapQuest (UK): 204.7 miles, 7 hours 23
MapQuest (US): 295.84 miles, 4 hours 52
They are all using the same route (A1 all the way up) except the RAC one, which turns off at Darlington to use the slightly more direct A68.
Personally I suspect the MapQuest US one is closest to the truth. How MapQuest UK manages to chop 90 miles off the journey and still add 2½ hours extra is beyond me. That's an average of about 30 miles per hour.