IGPOTY is a prestigious annual pay-to-enter competiton sponsored by the Royal Photographic Society, Epson, Calumet and several others. IGPOTY proclaims on the homepage "International Garden Photographer of the Year supports the Photographers' Bill of Rights and is recommended by Pro-Imaging."

IGPOTY is not organised by the National Trust, but a special section of the rules, section 12, applies to NT properties. It looks as if the NT has had more than a little influence on this section:

12.1 All photography at National Trust properties other than that for private and personal use or for entry into approved competitions is strictly prohibited.
12.2 Photographs taken for private and personal use may not be used in any other context or submitted to any photo libraries or on-line agencies or sold directly to any image buyers. Photographs taken for entry into competitions may not be used in any other context, including submission to any photo libraries or on-line agencies or sold directly to any image buyers, other than those confirmed by the competition organizers.
12.3 Photographers must obtain a National Trust property release form by downloading here. The photographer must print out the property release form which must be signed by appropriate National Trust staff at each of the properties where the photographs are taken.
12.4 Photographing people at National Trust properties must be undertaken with due consideration for their safety and must not interfere with their enjoyment or purpose. An example of a standard model release form is available for download.
12.5 People visiting National Trust properties are on private property and as such have the right to refuse permission to photograph them.
12.6 Every person in a photograph must sign a model release form, or have the form signed on their behalf if they are under 18 years of age, by a parent or legal guardian.
12.7 It is the photographer’s own responsibility to ensure they have explained to subjects the purpose of their photography and to explain how the picture may be used. The National Trust does not accept any responsibility for photography of people undertaken at its properties.
12.8 Terms of entry to every National Trust property should be strictly adhered to by the photographer, paying entry fees or showing an appropriate membership card applies.

This is madness. It is nonsense to suggest that you cannot photograph people on private property without permission : think CCTV. Think every snapshot you've ever seen taken in a park, on a beach 'owned' by the local council. Anywhere there is not a reasonable expectation of privacy is OK in UK law. Only amateurs taking photos for personal interest may take photos on NT property at all, but must then get property and model releases signed, the sole purpose of which is to allow NT to impose their loony terms. And having done all that, you can only enter competitions the NT approves of. Will anyone bother to read or understand these rules, and why would anyone bother to enter photos (and pay the fee) taken on NT property when it's such hard work?