Buying two tickets that cover the same journey in segments — e.g. London to Reading, then Reading to Bristol — can cost less than one ticket straight through, because fares are priced per operator section, not by distance. You stay on the same train; you just hold two tickets instead of one.
Advance tickets are tied to one specific train and go on sale roughly 12 weeks ahead, usually getting more expensive as the date nears. Off-Peak tickets cost more upfront but let you take any train outside peak hours that day. Booking Advance only pays off if your plans are fixed.
A 16–25, Senior, or Two Together Railcard costs around £30–£35 a year and knocks a third off most fares. If you'll take more than two or three return trips a year, it typically pays for itself on the first or second journey.
The cheapest Advance fares tend to release in batches and sell out by price band, not by time — the first ones sold are the cheapest, regardless of how far ahead you book. Checking right when tickets release (about 12 weeks out) usually beats checking the week before.