travel from Oxford to London-Charing Cross

I don't think your train ticket will be good on the Tube. And I don't think you're allowing enough time to deposit your luggage at Charing Cross and walk to Westminster. I suggest that you take the Oxford Tube (bus) from Oxford to Victoria station. On Sundays buses run at least three times/hour. The fare is £14. Deposit your luggage at Victoria station and take the Tube to Westminster. Or walk a couple of blocks and deposit your luggage at Victoria coach station. The rates are much cheaper than at Victoria train station.

Sylvia, according to National Rail Enquiries, that Advance ticket is £5 if booked to Paddington and £8 if booked right through to Charing Cross. The journey planner suggests a route via the Bakerloo line, so yes, it is valid on the Tube if that is what you book. The extra £3 is more than you would pay with a pre-paid Oyster card. I haven't checked the coach schedule that Tim mentions, but that would simplfy things. The walk to the abbey is about the same distance from Victoria or Charing Cross, 15 minutes, less if you step out briskly.

The X90, the 'other' Oxford to London express coach service now operates from Victoria Coach Station so is probably more convenient if you want to use the left luggage facilities there. Prices are generally identical and they leave from adjacent stands at Gloucester Green Bus station. The journey time is 100 mins notionally - probably most likely to be right early Sunday. The 8:45 train can be packed if there is anything of any substance happening on London that day - and I mean packed from Oxford, let alone elsewhere en-route. I speak from experience.