There are no terms requiring the customer to accept the variation, so let’s ignore all that. The CRA does not give any right to rescind or repudiate the contract, so that’s no help. The contract itself has no such terms either (contrast with, for example, package holidays, where the contract normally has a right to cancel for a “material change”). We therefore need to fall back on the common law.
Here we are dealing with an anticipatory breach of contract, but not a repudiatory one. To be a repudiatory breach, the breach would have to deprive the innocent party of the substantial benefit of the contract. The substantial benefit of a normal train ticket is travel from A to B. The non-provision of catering on board the train does not deprive the passenger of the substantial benefit of the contract. It follows that the appropriate remedy, if any, is damages at common law for breach of contract, or in the alternative, an appropriate price reduction under the CRA. The amount of any damages or reduction would be for the parties to agree, or the courts to decide on in default of agreement.
(By contrast, were we discussing a rail tour or similar arrangement where the journey is circular and substantial meals are included in the ticket price, I would be considerably more likely to conclude that the non-provision of that on-board service deprives the passenger of the substantial benefit of the contract and gives rise to a right of repudiation.)
In the present situation, however, it is not correct that the theoretical passenger has the right to refuse the variation and repudiate the contract (or obtain a free ticket change);
however, as I said previously, I anticipate either of these options would be offered by the TOC if asked politely.
You can read more on the topic of anticipatory and repudiatory breaches at
https://www.inbrief.co.uk/contract-law/anticipatory-breach-of-contract/