Southern, though they have a terrible train service, offer a refund for
- Anytime
- Anytime Day
- Off-Peak
- Off-Peak Day
- Super Off-Peak
if you don't collect the tickets. I've used them for booking multiple tickets on different services when I wasn't sure I could time it precisely, and refund the ones I don't need. It's important to not confuse the booking numbers and collect a ticket you don't want by accident.
Sadly Southern have recently switched to the god-awful booking software that wastes most of the screen on a pretty useless big map and no longer let you book bikes, which has reduced my use. Checking
www.AtoB.org.uk rail zone will say which sites still let you book bikes when needed and I prefer to reward those sites by using them most of the time.
The Trainline also doesn't let you book bikes and I thought it still charged you a booking fee so it's never the cheapest way to buy?
A good ticket office is often the best way because they know most of the wrinkles, such as the boundary extensions mentioned above or whether it's worth getting a discount railcard or network card, but of course sometimes you get a worker who doesn't care and will sell you what you ask for instead of the cheapest thing that does what you need. There are also some interesting new ticket sale websites like
www.TrainsCanBeCheaper.info for longer journeys.
But as mentioned above, for some short journeys prebooking makes no difference because there are no Advance fares on some routes - you can check what might be possible at
www.brfares.com amongst others. Also, when it does, the best prices are usually 10-12 weeks ahead, locked to a specific time, non-refundable (note they're not on Southern's list above - some of them can be changed to another time by paying the difference, though), and prices go up as the departure gets closer, usually maxing out on the same day.
Upshot: when I can't get to a ticket office (my local station has never had one in my memory, but the two next nearest do), I've gone back to using GWR's website most of the time.