I'll be leaving from Heathrow about 7:30 on a Friday evening, wanting to get to Gloucester as soon as. Do I take the M4 via Swindon or the M40 via Oxford? On googlemaps, the two routes look very similar in terms of distance and (off-peak) travel time -- but I don't reckon this will be off-peak, will it? Is one way notoriously worse than the other? Thanks in advance.