But in the event of a cancelled train, the OP accepts provisions in the contract that say:
He can abandon his journey and claim a full refund, or
Can travel in full and claim Delay Repay for the delay at journey destination.
Neither appears to have happened here. The OP didn't abandon his journey.
Firstly, buying multiple tickets doesn't automatically mean you are making one journey - NRCoT 14.1:
Unless shown below, you may use a combination of two or more Tickets to make a journey
"May" means this is a right which can be exercised at the sole discretion of the passenger.
The passenger could quite rightfully decide just to use the Derby to Crewe ticket, and to get a "change of mind" refund on the rest of the tickets.
They could also use the DBY-CRE ticket and then give/sell the other tickets to a friend.
It doesn't matter - the passenger can determine the journey as they desire. The fact that the tickets were all bought in one transaction (which is, I think, the reason why the retailer is being difficult here) doesn't change that.
Furthermore, as the NRCoT say:
If things go wrong, in addition to the industry arrangements set out under these Conditions and Passenger’s Charters, you are also able to rely on statutory rights.
So it would be quite wrong to suggest that, just because the NRCoT bestow contractual rights such that passengers
may claim a refund, or
may claim Delay Repay, this means that there is no other remedy available to the passenger.
Section 50 of the Consumer Rights Act sets out that any information provided about a service is binding, if the consumer took it into account when making their purchasing decision. Timetable information is undoubtedly taken into account when buying a ticket, and accordingly is a binding term of the contract.
Ss.54 and 55 provide that the consumer has the right to a price reduction (i.e. partial or full refund) where a service has not been provided in conformance with the contract, if repeat performance in compliance with the contract is impracticable.
If a train has been cancelled, repeat performance in compliance with the contract is clearly impossible - unless there's a later train that will arrive at or before the originally booked train's arrival. Accordingly, the consumer has the statutory right to a price reduction. The level of the price reduction is to be an "appropriate amount".
In the circumstances of a return ticket, where the return train is cancelled and the passenger makes their own way back, an "appropriate amount" would seem to me to be 50% of the fare. Of course the TOCs and retailers may have other views on this!
S.54(7)(b) specifically alludes to the common law right to "recover money paid where the consideration for payment of the money has failed". This is another option available to the OP, though it leads to essentially the same outcome as a claim under s.55.
In other words, "the NRCoT don't say you can do this" is almost completely meaningless. The NRCoT don't purport to limit your rights - and couldn't do so even if they wanted.