I'm scheduled to land at Heathrow at 12:10 p.m. on Monday the 16th. I'm heading to Bath Spa. I am taking the Heathrow Express to Paddington Station and then the GWR to Bath. (I know there are perhaps cheaper and easier ways to do this.)
Since I don't know how long it will take to get through passport control, I don't know which Paddington to Bath train I will be able to catch. Maybe as early as 2:00 p.m. Maybe as late as 4:00 p.m. Since the advance ticket price varies depending on departure time, should I book, say, the 2:30 out of Paddington for about 34 pounds, knowing there is some chance that I will have to take a later, and more expensive, train? Can I just pay the difference if I don't make the earlier train? Or should I just not worry about buying an advance ticket and just pay the fare for the train that I'm able to make?
Note that I already have my Heathrow Express ticket.