They won't be doing 80...or 70 or 60
First they will be in a queue to the Headington hill roundabout off the A40/M40 route.
Next comes the Oxford northern bypass which is invariably queued at the Banbury and Woodstock road roundabouts: so bad is this area that it can be better to go round the entire ring road in the opposite direction to the south and up the west side - which becomes the A34 so has the almost daily crash on it. Much of the ring road has a 50 mph limit on it.
Finally there is the dual carriageway route (A44) from the Oxford ring road to the airport - also with a 50mph limit.