Just to clarify John's point,
figure out which lines & times an advance-purchase train ticket would be good on if we didn't make the original train we picked.
The various companies work together to provide a unified service. Some routes between two points are longer, some are shorter, to allow for the various stations on the way, and some trains call all stations and some are express. This is all detailed as you drill down in the list of trains offered at National Rail Enquiries or the websites or apps of the various train operating companies.
If you had purchased an "Anytime" ticket either in advance or on the day of travel (same price, the highest, regardless) and missed the train you had planned for, you could board any earlier or later train of a similar type going to the same place. The actual ticket will have any specific restrictions on it, such as via X or only valid on Y train company. You don't need to do anything, just meet any restrictions and hop on. It does need to be on the same day, up until 3am the next morning.
Less expensive tickets have more restrictions. "Off-Peak" tickets are just that. They are similar to Anytime tickets in that you can take any train meeting he restrictions, but now only at non-rush hours times. The definition of Peak varies from one part of the country to another, and from one line to another, but generally from about 0600 in the morning until 0930 or 1000; and from about 1630 to 1900. You wouldn't want to be on a Sardine Express anyway. Those time restrictions are tightly enforced. It will be clear when you buy the ticket.
The cheapest tickets with the most restrictions are "Advance" tickets. The name is the same as one of the restrictions. They must be purchased well in advance of the travel, up to 90 days ahead. They are capacity controlled and the further out, the cheaper. When they are gone they are gone. They also have another very strict restriction - you MUST travel on the exact train you have bought at the exact time, in some cases in the exact seat you have bought, and they are invalid on any other train at any other time, and are non-refundable and it will cost a lot to change them at the ticket window in advance of the departure. Don't ever get on, or try to get on, a different train than the one named on the ticket - it is worthless and you will be treated as if you have no ticket and perhaps ge given a fine.
It sounds very complicated but in practice it is quite simple.
You need to decide if you can without doubt make the specific train on an Advance ticket, otherwise pay more and get the appropriate Off-Peak or Anytime ticket.
I hope that helps.
