I'm thinking start on the standard route, and if those lanes are bad, step off onto the A420 somewhere.
From Swindon across to Malmesbury probably the standard lanes, but there's a B road option.
From Ciren, almost for sure, the Stow road, and from Chipping C also. If the A44 is too scary, I can cut over to the Kidlington road, I guess.
Take extra rear lights and the tabard.