The National Conditions of Carriage actually permit breaking an outbound journey, provided the ticket remains valid for the continuation. Outbound tickets are usually only valid on the day, so in that case you can't continue the journey the next day. But a regular standard (or first-class) ticket on a train that's not the last train of the day can be broken on the outbound journey.

For most tickets.

Some tickets, such as Advance and some Super-Off-Peak, have additional conditions. However, I do have a letter from a train company saying that they were wrong to have charged for an extra ticket, and wrong to have tried to start proceedings.

So breaking an outbound journey can be legal, and in such cases buying an over-length ticket is possible. It's circumstances like these that led in part to some of the specifics of the recent shake-up of the fare structures.

Huh. I wonder if it would be cheaper to do a ticket like

start -> actual destination (stay on train) -> overshoot destination (get off) (get back on) -> (turn around; begin return trip) -> actual destination (get off) (get back on) -> start

?

Seems utterly unintuitive, and it probably is, but I am curious.

That can be possible, and I have done it.

Part of the reason for all this is that the train system has been broken up into a collection of franchises. Some companies will then incentivise some journeys, and so the fare doesn't always reflect the time or distance of the journey being made. As a result you can find and exploit anomalies, but only if you're willing to spend the (sometimes considerable) time and effort.

And how do they enforce it? Chain you to the train?

No, they fine at exit.