To state the obvious, if the government had not thought it prudent to retain Open Access, we would not even be in this ridiculous sitation.
That said, one wonders how GBR would approach a choice of either running a service to time but without toilets, or just cancel it. Which under GBR, in the main, you would assume would only happen at very short notice or indeed part way through a journey.
There is only one right answer, if your aim is public service in 2026. You run the train, to time, so as not to inconvenience those who have paid for it and can reasonably know they will not need an on train convenience, and offer a full refund to all others.
And furthermore, appropriate compensation too, for the literal inconvenience in a situation where it's a virtual certainty the issue was avoidable.