The outward route suggested above is fine - I know the roads fairly well and you are right to say it's not flat. For this reason I intend to go back by the easy route: Headington, cycle path beside the A40 to Wheatley, then the A40 from there eastwards has very light traffic as far as High Wycombe (because it's right next to the M40). Even after H.W. it's not too bad up to Denham, and after that you're almost back into the suburbs and it's just a question of gritting the teeth. It's also possible to turn right at Stokenchurch (the top of the big hill) and go back to Marlow via Lane End. There are various easy routes back from there to Staines where again you're pretty well into London. I could do either route, but my first preference would be H.W. and Uxbridge since I'm going back to Willesden. All this assumes we don't have a strong east wind, in which case I'm staying overnight in Oxford.
Just a little note about the return.
The outward route suggested above is fine - I know the roads fairly well and you are right to say it's not flat. For this reason I intend to go back by the easy route: Headington, cycle path beside the A40 to Wheatley, then the A40 from there eastwards has very light traffic as far as High Wycombe (because it's right next to the M40). Even after H.W. it's not too bad up to Denham, and after that you're almost back into the suburbs and it's just a question of gritting the teeth. It's also possible to turn right at Stokenchurch (the top of the big hill) and go back to Marlow via Lane End. There are various easy routes back from there to Staines where again you're pretty well into London. I could do either route, but my first preference would be H.W. and Uxbridge since I'm going back to Willesden. All this assumes we don't have a strong east wind, in which case I'm staying overnight in Oxford.