If your ticket type is "Advance" (this will be printed prominently on the ticket) it means any reservations you have are mandatory, and you must use the train booked in the reservation, unless a delayed train causes you to miss your booked connection (in which case you will be allowed on the next train). For legs of your journey without specific trains specified on the tickets and reservations, you may use any suitable train. If, on the other hand, your ticket type is "Super Off-peak (Day) Single/Return", "Off-peak (Day) Single/Return", "Anytime (Day) Single/Return", you can use any train, provided you comply with peak-time restrictions if your ticket is off-peak or super off-peak (newer ticket styles have a web address you can visit to discover what these restrictions are; older styles have a two-character code printed near the word "Validity", in which case you can go to http://nationalrail.co.uk/XX where XX is the two-character code). If you tell me the exact journey you're making and how much you paid I can probably find this out for you now.
Going to Swindon, the trains currently in use are from the 1970s and have manual doors which can take a bit of getting used to for tourists. They're soon to be replaced with newer ones, so depending on when you're travelling you might not have this problem. If you find yourself at the front of the queue to open the door on the train, you need to lower the window, and use the door handle on the outside. Wait until the guard has unlocked the doors first (a "Doors unlocked" light will switch on inside, and a large orange light on the outside of the train). Similarly, if you're the last person getting on or off the train, slam the door shut behind you and save the dispatch staff a trip down the platform.