Oxford -> Bourton-on-the-Water (very posh and Cotswoldy) -> Cirencester (also taking in Chedworth Roman Villa perhaps) -> Bath -> Cheddar Gorge -> Exeter (maybe) -> through Dartmoor -> south coast of Cornwall (e.g. Mevagissey)
avoid Bristol (crazy traffic), Plymouth (horribly run down) and Truro (meh)
you could come back along the south coast (the Jurassic Coast east of Weymouth is nice), and break away to come back through Winchester (seat of Alfred the Great)
all that lovely relaxing driving on ancient road systems with far too many cars on the road is just what you need before a few days in one of the busiest cities in the world