Linbow, I am so sorry to hear this. It seems extremely unreasonable and how disappointing for both of you, as long standing members.
I think the law (the Disability Discrimination Act) requires service providers to make ‘reasonable adjustments’ for disabled people. This sounds like the NT are not doing so. You could write again, as a member and request a copy of their Equality Policy and warn them that you feel they are not making their facilities accessible to your husband, by their inflexible approach to booking.
Conversely, perhaps you should book and then not turn up, if your DH is not up to it. I accept this ‘goes against the grain’, but I think their intransigence might provoke such a reaction in me.
Hope things get sorted out for you both. Best wishes ?