What's the reasoning on that though?
Delay Repay is based on
actual delays incurred, not hypothetical alternatives.
Arguably it should also account for having to leave earlier due to disruption - but as it stands, it doesn't.
You would have to raise a claim under the Consumer Rights Act (or similar) though customer services. You may alternatively want to claim a partial refund on your ticket commensurate with the part of the journey you made by road - although claiming such a refund is not something that industry guidelines and procedures readily cater to - and it is not always obvious how any such refund would be calculated.
The delay to the intended itinerary would have been around four hours had I stayed to relying on a service they didn't provide.
Unfortunately the rail industry has no interest in thanking you for avoiding a large compensation bill. Indeed on several occasions I've been refused travel on earlier services during disruption, even though this would have avoided Delay Repay liability for the train operator!
On the whole, the rail industry is far too myopic - obsessing about the letter of restrictions and conditions rather than looking at the bigger picture in terms of reputational damage and financial losses caused by enforcing terms in a "jobsworth" manner.
But sadly that doesn't change the legal position, which is that Delay Repay isn't due if you leave early (or "under your own steam") to arrive on time.
Is there not a case that the cost was been more than the ticket and therefore reasonable costs should be refunded?
I think the rail industry's view would be that you could have avoided those costs by waiting for the next suitable train, and since it was not a matter of otherwise being stranded, the costs were thus incurred "unnecessarily". Of course that ignores the fact that people book particular trains to be at their destination by a certain time, and arriving hours late isn't a feasible option.