If you can get down from Oxford to around Wantage - you can pick up the Ridgeway - which will take you west towards Marlborough way, then scoot down to the Kennet&Avon.
Or a flatter option - if you get down from Oxford to Reading way (Thames path?) you can cycle the Kennet&Avon (towpath obviously) down to Bristol.
Not sure where you go from there.....
Wanted: Trek XO2 54cm
Running:- Orange AluO; Orange Gringo; Orange Evo2; Trek Madone 6.9 SSL; Trek 1500 SLR
In Progress:- Orange Elite; Trek Elite 9.8
Selling:- Orange Sub3; Orange O2
Sold :- Merlin Titanium; Specialized 99 FSR Elite; Orange Mr XC; MountainGoat WhiskeyTownRacer; Orange AluO