Well I will still be going to Fountains Abbey. Not sure about the 13th of Jan, but sometime in the next few months.
I have taken two shots from there, and have put them on my flickr gallery, as have countless thousands of others around the internet.
I think it's whether or not you count sharing them on a website as publishing (I wonder if there's case law to define putting on a website as publishing).
I think that's where NT's problem lies. They don't want possible sales being lost.
So if I was to put up a fullsized image on my flickr account, then a potential customer would possibly buy it from me, instead of directly from the NT, which is where their want to protect themselves comes in.
I doubt they would have a problem with us sharing websized shots on the forum or personal galleries.
I wouldn't sign that form either. I too don't sign a free copyright release of my work, even if it did include due credit, unless it was for a specific publication and instance that I agreed to.