I don’t understand. Wouldn’t most fare dodgers buy no tickets, not just buy a single ticket for part of a return journey?

What’s the reason that people try to dodge fares by buying a single ticket? Is it because they know when and where tickets are likely to be checked?

I believe the PC is hinting about station barriers. Major stations need you to scan a ticket (or con a guard) to get off the platform or out of the station. Minor stations sometimes don't - and if it's your station you'll know.

I think the real reason is less sensational: trains that go full in one direction still have to go back in the other direction whether they're full or not.

The real reason is the government has set return fares and let private companies freely set the price of single fares.