In reply to Toby S:
Depending on time of day and whatever roadworks/traffic jams etc there are there isn't much in it.
The A34 is a pretty good road and doesn't have any road works on at the moment as far as I'm aware but the area near Oxford is a total pain in the backside at rush hour.
The A36 is slow and meandering but not too bad with traffic although I know a lot of people who say Salisbury is bad, I've never had a problem. Bath on the other hand should be avoided at all costs; I wouldn't drive through Bath before about 10pm, otherwise you'll have a long time to take in the delightful architecture.
Personally I'd go M40, A34, M3, M27 as the most reliable and fastest. The M3 is a bit of a pain Friday evenings but otherwise pretty good.
Depending on if you are coming down from Birmingham or actually coming from further North I'd suggest that the M1, A43, M40 dogleg can be a good option rather than doing battle with the M42. You can skip the M40 altogether if you go straight over the roundabout at Cherwell Valley services which is a good option in the event of an accident (best done with a navigator). Bicester should be avoided during shopping hours as the retail park can create awful traffic.