Are you actually entitled to a part-refund of a season ticket if you decide not to travel on a particular day? Holiday, sickness, strike, work from home - that's not the TOC's fault ....
If the reason for not travelling is nothing to do with the Railway (i.e. such as you being on holiday, or ill, or working from home) then I would agree but if you choose not to travel due to disruption? I'm not sure it's quite so clear. On any other type of ticket (such as a walk-up or Advance) if you choose not to travel when the service is disrupted you are entitled to a refund if you choose not to travel. The NRCoT says:
30. Your right to a refund if your train is disrupted and you choose not to travel
30.1. If the train you intended to use is cancelled, delayed, or your reservation will not be honoured, and you decide not to travel, you may return the unused Ticket to the original retailer or Train Company from whom it was purchased, where you will be given a full refund with no administration fee being charged.
This Condition applies to all Tickets, including Tickets (such as advance Tickets) that are otherwise non-refundable, and also applies if you have begun your journey but are unable to complete it due to delay or cancellations and return to your point of origin
Indeed it actually says "This Condition applies to all Tickets"!
However I'm not sure what the "refund" would be in this case. I'm assuming that the OP wishes to continue to carry on using their season ticket so they can't apply for a refund of the whole ticket (like you would with say an AP if you abandoned your journey due to disruption). My gut feeling is that this scenario wasn't really thought about when the NRCoT was drafted so doesn't actually cover it.
That being said I do wonder if such a claim would fall over on the "unused" requirement. Presumably the season ticket has been used for travel on previous occasions which would, to my mind, mean that Condition 30 could not be activated even if it hasn't been used on the day that a passenger was intending to travel and then didn't due to disruption.
My suggestion to the OP would be to write to the relevant TOC, enclosing a picture/photocopy of the season ticket and explain what happened and ask that they consider what compensation is due. I suspect it will just be treated under their Delay Repay scheme but you never know!