The Copyright Action website has published a comment on the possible reliance of the National Trust on a 1965 schedule of Byelaws to ban commercial photography. Their conclusion is that this regulation (similar in legal basis to byelaws passed by local authorities, town or county councils) was intended to prevent itinerant photographers setting up to photograph visitors – the old tintype seaside trade.
http://copyrightaction.com/forum/national-trust-byelaws-in-a-twist
Here are the Trust byelaws in full.