I can sort of understand it if you are taking photos of one of their big properties such as Montacute House or something, but one would hope common sense would prevail where it is open land and there is no entrance fee!

Perhaps, in light of this, you could suggest to the NT that your photos highlight the Trust's important work in conserving our prehistoric heritage and in turn it would be a good free advert to promote membership. Or suggest you could put a dedication to the NT in your book and contact details for those wishing to become a member. To me this seems a reasonable compromise, but sadly people are too self-obsessed and selfish these days.