When the Passengers Charter was introduced onto the railways in the 1990s by British Rail it offered standard compensation for passengers. The highest level was where one leg of the journey was delayed by more that 120 minutes, and yes it does happen, the passenger could claim back the total cost of a return ticket and not just the leg of the journey that was delayed.
Now that was in the day when virtually everyone making a return journey bought a return ticket.
In recent years train operators have been advising us that the best value may be to buy two single tickets, one for each leg of the journey.
But by doing so the train operators have been able to save on paying out compensation. This has been possible for them because the two single tickets are just that, single tickets. You may be making a return journey but in the eyes of the train operator you are making two single journeys, albeit in opposite directions. A full refund will be made in many cases if the train is more than 60 minutes late, but should it be over 120 minutes late then the refund remains the same. Your contract is for a two separate single journeys.