Ok i assume that u wont have much trouble with Oxford and Cambridge being fairly large towns.
In regards to stone henge, if you are going for the stones u may want to consider Avesbury which has a village in the middle of the henge instead. (big plus is its free to walk amoung the stones) (Swindon is the nearest train station and then a bus )
for rail try http://ojp.nationalrail.co.uk/planmyjourne...9_2005624142147
and put in start and end and it will tell u how long it takes and what trains to catch. I find it alot easier to use than the bus, but it depends on how many changes u need to make for each journey.
If u were to travel from Cambridge to Stratford upon avon by train would take about 4 hours but u need to go via Birmingham or london.
To get to stonehenge, Salisbury is the nearest train station and i'm not sure but its pretty likely there will be buses from there.
for info on how to get there.