There is a good reason the return is only £3 more. It's so fare dodgers who buy a single have still basically paid the full fare.
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 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.