As a member of the Trust I've never had a problem taking photos for my own private use. Indoors or out.
Extract from the NTs web page
http://www.nationaltrust.org.uk/main...qs-visitor.htm
"Amateur photography (including filming) without flash is now permitted in historic interiors at the Property Manager’s discretion. As with outdoor photography, any photographs taken are strictly for private use, and enquiries about selling or publishing photographs should be directed to
photo.library@nationaltrust.org.uk. However, visitors must be aware that at some places, there may be copyright issues, and further permissions may be required in respect of collections not owned by us. In these situations the Property Manager decision as to if photography is allowed is final"