Written August 2026. Not legal advice, just the rules in plain English.
Say your ticket is Reading to Cardiff, but a friend can drive you as far as Newport. Can you just hop on there? Usually yes with a flexible ticket, usually no with an Advance. That is the whole answer, and here is the minute of detail.
With Anytime tickets, and most Off-Peak ones, National Rail's conditions let you start, break or end your journey at stations along your ticket's route. Board at Newport with a Reading to Cardiff ticket and you are simply using less of what you paid for. The exception is when a restriction printed on your ticket says otherwise, so give it a glance before you rely on this.
An Advance ticket is locked to the exact trains you booked, and it is not valid if you start partway along the route. Board late on an Advance and you can be treated as having no ticket at all, which is an expensive way to save money. If your plans have changed, change the ticket before you travel.
If a lift is getting you partway, the simplest thing is a ticket from the station where you actually board. No rule games, nothing to explain at the barrier, and it is often cheaper than the through fare anyway.
This is exactly what Partway does. Tell the free iPhone app about the drive, and it prices real tickets from every station along the way, so you board with a fare that fits.