A good start point could be to look up the route used for the annual London to Oxford bike ride. That would avoid most of the busier parts and then deviate from that route as you get close to Oxford by dipping down between Oxford and Abingdon. The last section from there to Cirencester should be relatively easy to find quieter roads.
If you want to put more work into the planning and don’t mind a less direct route then it certainly possible to plot out an even quieter route but the London-Oxford seems like a good start point. The ride has started in Ealing and Isleworth in different years so that should give you a couple of options.