You dont pay for train tickets to stand or sit on the floor...
That's exactly what you do do, you pay for a journey on the train; the fare doesn't include a payment for a seat or even entitle you to have one. Booking a seat is free.
I said on the other thread, that people needing a priority seat can book one by calling the PassengerAssist line and asking for one along with any other help needed. Help can also be booked by filling in the form all ToCs have on their websites.
If you just want a reserved seat but not specifically a priority one, call Customer Services if you've already bought your ticket, or make a reservation when buy your ticket. If you need specific help do ask for it, most ToCs do their best for you. I'm railway retired staff so I've done a huge amount of rail travel for work, and a lot for leisure as well as I took early retirement back in the day when retiring staff still kept their full travel benefits having spent many years working for the national railway.
There's more help available from the train companies than you might think, it just needs you to be able to plan a bit ahead to take advantage of it. After you've booked help you'll get a detailed email from the ToC you can show to any stroppy sod sitting in your reserved seat. The station staff at your departure, interchange and destination stations should, and in my experience usually do, know about your booking and are looking out for you at each stage of your journey. I have experienced some failures but each time, all three of them at Reading, Great Western Trains sent me home after midnight, to near Bath, about 75 miles, in a taxi because the staff at Reading didn't have my booking.
Of course I know about the staff shortages, but the ToCs have a statutory duty to help rail travellers who need it so if you book it and it isn't satisfactory, do complain about it and let them know where they need to improve. Failure to give the help needed might be grounds for complaint under the Equality Act 2010 so the ToC should take notice of complaints.