I don't need to define what a "journey" is; what matters is that passengers
may combine tickets for one journey.
The passenger
may choose for this to be a CLS-NMP journey, if they wish.
The passenger
may break their journey if they wish to do so, so that is what it is. You can't easily define "journey" but rights have been given to passengers that state they
may use two or more tickets for one journey.
As XC do not sell Weekend First fares for anything other than the portion of the journey that is with them, then in this case it would be York to Birmingham. If a through Weekend First fare was available then that should be offered to the customer.
The ultimate irony is that XC do not like people purchasing a combination of tickets, so XC insist on refusing to sell a through upgrade for their journey, and attempting to sell them a combination of upgrades to get back at them
Passengers are allowed to use two or more tickets for one journey, and that is the key issue here.