It's 6 hours.
Easy enough to switch from car to car - after 5 minutes there's not much to it.
Watch out for cameras on the M4 around newport and cardiff - SPECS - stick to 50 mph.
Other than that, it's an easy drive really. From Oxford you've got 2 options, either back to the M4 or go cross country to the M50 and then down the A449 onto the M4. A34 back to the M4 is probably easier though.
I've done over 6 hours of motorway in the Alfa non-stop, not even stopping for petrol, and In traffic a lot of the way. This in a car which is doing over 3500RPM at 70, and starts to get noisy past 2500RPM.
SPECS
I drove the Mercedes on the way back... it was fine actually(
I didn't realise there was SPECS there, I thought it was just signs to do 50?
I've driven through there faster than that with no ill effects? Is that recent?
Just watch out for the new specs in the M4 around Newport, as its a 50mph limit now, even though the road works are not in the area!