Here is the route we've taken in the past.
Start in at the Runnymede Hotel on the A308 right by the M25 at Staines. First it's off through Windsor, then A4130/A4074 through Henley to Oxford and a pause at the Oxford Services. This is the services at the junction of the A34 and A44, I think it's called the Peartree Interchange. We then take the A44 to Chipping Norton, A3400 through Shipston-on-Stour to Tredington, A429 past Wellesbourne to Warwick, the A4177/A452 through Hatton and Balsall to the A45.