The current refund/compensation system is complex and causes confusion for staff and passengers alike.
If a passenger chooses not to travel, they need to seek a refund from the retailer. They are entitled to a refund for all ticket(s) used for the journey, and this should apply equally to both portions if this is a return journey. This is problematical if a customer has used multiple retailers for their journey, as retailers for parts of the journey that are unaffected may refuse to pay out.
If a passenger attempts to travel, but abandons their journey and returns to the point of origin, again they are entitled to a refund, but this also needs to go back to the retailer. As above this can cause issues if there are multiple retailers involved. To make matters worse, some train company staff refuse to accept that a passenger as a right to abandon their journey and may be obstructive in allowing passengers to return to their point of origin without requiring them to pay for yet another ticket. Some retailers refuse to refund tickets that have been scanned/marked/stamped, which of course a ticket for a journey that was abandoned part way through may well be!
If a passenger does make the journey, they are entitled to delay compensation from the train company whose train caused the initial delay.
A passenger who abandons their journey part way through may find it harder to get a refund than a passenger who stuck with the journey and arrived late at their destination.
Also if a delay occurs and the journey is no longer viable by train, a passenger who uses another method of transport may be told by a train company that they are entitled to absolutely nothing. For example, a Season ticket holder whose train is cancelled who then gets a lift with a work colleague instead. This is because delay compensation is not considered to apply (as they did not make the journey) and therefore refund arrangements apply, but they are told that their Season ticket is "already discounted" and therefore no pro-rota refund applies. No other industry treats its most loyal customers as badly as this, yet some train companies routinely do exactly that.
The result is that passengers can be sent back and forth between retailer and train company and can be left out of pocket as well as potentially giving up hours of their time writing letters. There have been numerous instances of railway staff giving incorrect information to passengers who then find their claim rejected because it went to the wrong place.