I really don't think a ticket to Cholsey is going to be accepted to Reading. It's not valid because it's doubling back, which is forbidden except in the case of group stations for interchange (see RG) or where an easement exists (see RG again).
The only easement that may be of use is this:
"Journeys via Swindon and Didcot or via Swindon to Didcot and routed Not London may double back between Reading and Didcot. This easements applies in both directions."
So, for example, you can use a ticket from Chippenham to Oxford via Reading. Although the "Not Reading" fares are considerably cheaper....
You can double back if it's sensible to do so - the RG doesn't quite allow for all sensible doubling back possiblities, but given the distance involved in this case, and given the good connections available at Oxford, I don't see any justification. Saying that, some guards will be fine with it though.
Edit: While Cholsey is no use, Goring does have a "Reading" route option, which explicity permits doubling back via Reading. This is a few pence more than a ticket to Reading.