I'm not sure if it's lawful for them to insist that people with medical conditions or anxiety wear a mask while in the UK; I'd have thought not, but once you cross the border, I'd assume French law would apply (perhaps someone can confirm).
If their T&Cs are at odds with the law, then it could make an interesting legal case on the basis of discrimination but I have no idea what the prospect of success might be.
Not on topic for this thread, what
@yorksrob says is true of Northern Rail; let's discuss in a more suitable thread as it's not directly related to face masks (other than indirectly in that the fact that the wearing of masks is likely to result in the abolishment of social distancing on trains, which in turn will presumably result in the message from TOCs changing)