Oh sure...... Park the car on the side of the road for some ne'er do well to steal. Great idea, that.
First try actually getting to Oxford - or preferably past it! Driving to Didcot is on back roads, so the trip is reasonably easy. The useless, non-integrated travel system in the UK doesn't have buses which meet trains either, so if I walked to the bus stop it'd be pot luck as to whether the bus could get me to the Cotswold line station on time....and there are few trains from there. Oxford itself? YGBSM!!
Back to the topic of Forces rail travel, one used to be able to buy 'Forces Leave' tickets at a substantial discount (about 33% off) but that perk disappeared when the military salary came in.