The Senior Railcard = 1 each at £30 per person & can be used before 9.30am on weekdays. The 2 Together Railcard = £30 for the 2 of you but is not valid before 9.30am on weekdays. (Can be used anytime on weekends).
So, you want to land at Heathrow and travel to MIM. You can do this by train by doing this on the following site. Put in for Heathrow to MIM and click ADVANCED OPTION & tick NOT VIA LONDON. Then just follow the instructions that it gives you. https://www.traintickets.com/?/
I am not sure if you are staying in MIM or some other Cotswold village or whether you plan to stay in Oxford? If you are staying in MIM then you can pay on the day for the short train ride to Oxford as a day return ticket. Also note that if staying anywhere near Cheltenham that direct trains go from here to York hourly.
For your longer journeys to the north, it is much cheaper to pre purchase Advance tickets for specific trains. If a strike happens, they will give you a refund or swop to another day if you so desire. Hopefully all the aggro from the rail unions will be over by September or earlier.
If you go to https://www.traintickets.com/?/ and check out the prices going forward you will see that August is now in darker print meaning that the Advance tickets are now available. However, note that September is still in faint type meaning that the cheaper advance fares have not yet been loaded and you would pay full whack if booked now.
I would not go from York to Heathrow prior to a flight - just in case something goes wrong. I would in fact stay in London for the final few days so you are just a short distance from Heathrow.
I have a way of getting you from MIM to Edinburgh for the same price as MIM to York - providing you don't mind the 6 hour trip as opposed to around 4 hours from MIM to York. Go to https://www.traintickets.com/?/ and put in for MIM > EDB for a date in August with a Railcard. You will see that a train leaves MIM at 10.19 and that you change trains (routes) at Worcestershire Parkway for one going to Nottingham (from Cardiff) . When this train arrives at Birmingham New Street, you change again for the 12.17 Avanti West Coast service to Edinburgh (which normally goes from platform 7). Now the website I have given splits the ticket up into sections for each leg and thus saves £75 on the through ticket- even though you are travelling on the same trains. This means that you can get from MIM to Edinburgh for about £46 each with the Railcard. Another advantage of doing it this way is that you travel up the western side of the country instead of go up and back down the same route on the East Coast mainline form London.
Note that if any connection is missed and it is not your fault, the railways are obligated to take you on the next available service at no penalty. You can also get compensation for late arrival. Another site that works out if split tickets are cheaper is:> https://new.trainsplit.com Note that these ’split’ sites take a commission of the saving achieved.
So, having done Edinburgh, you travel back to York and finally London. In the event that you travel directly from Edinburgh to London note that a new train company operates the route in addition to LNER (the main operator) - BUT they don't stop at York. https://www.lumo.co.uk/tickets/prices