The destruction of the environment by road users is growing daily and the only way to reverse that is with a publicly owned rail system run for the benefit of the public not private business.
I'm not sure that I follow how your reach that conclusion from the initial fact quoted. Whilst I agree that moving more people from road to rail, and encouraging rail use with an environmental as well as a profit bias would help, it's not the sole or only way.
For example:
As for ^20 fare plus ^4.20 for parking ...
I think you live in a location that has 2 buses an hour to Didcot station. And I'm pretty sure you're eligible for free local bus travel. So another way to cut the environmental impact of road travel is to make that road travel more efficient ... by having you take the bus which is already running (and it will save you 4.20 as well!)
As for ^20 fare plus ^4.20 for parking being a good deal I must have missed something or is the ^8.00 fare on the Oxford Tube a mirage? The problem of course is that the more passengers Oxford Tube attracts the more damage is done to the environment.
You are not really comparing like for like. The Oxford Tube fare is 16 pounds return; you have discounted that one based on the discounts you can obtain, but you have not discounted the rail journey similarly ... and if you're not going to use the bus to get to Oxford to catch the tube, you need to add driving and car costs to straighten out the comparison. Then you have the matter of the train taking 40 minutes from Didcot and the bus taking 100.
If you have plenty of time, and cost is important, you could make the whole journey by local bus (and then it would be at no cost to yourself, as the taxpayer would pick up the bill which would be around 60% of the bus fare)