I think the Animal Welfare Act 2006 would be the legal document ruling the treatment and housing of dogs.
Section 9 "Duty of person responsible for animal to ensure welfare" states:
(1) A person commits an offence if he does not take such steps as are reasonable in all the circumstances to ensure that the needs of an animal for which he is responsible are met to the extent required by good practice.
(2) For the purposes of this Act, an animal’s needs shall be taken to include
—(a) its need for a suitable environment
—(b) its need for a suitable diet,
—(c) its need to be able to exhibit normal behaviour patterns,
—(d) any need it has to be housed with, or apart from, other animals, and
—(e) its need to be protected from pain, suffering, injury and disease.
This is kind of a general introduction into the legal duties of any pet owner. The phrasing "by good practice" means that the current, most agreed-upon guidelines by reputable sources apply. That allows the guidelines to change without the law having to change it's wording.
Current guidelines for dog crates include these criteria:
- The crate must be longer than the dog from tip of the nose to base of the tail
- the crate must be higher than the dog's top of the head while sitting straight up
- The dog must be able to turn around
- The dog must be able to stretch it's legs out while lying on it's side.
Sources: RSPCA, Animalso, PreventiveVet, CertaPet
The Department for Environment, Food and Rual Affairs issued a Code of practice for the welfare of Dogs based on the Animal Welfare Act 2006 that defines some more details:
- Make sure that any place you leave your dog is large enough to provide, at all times, a comfortable area with effective ventilation and temperature control, and that your dog is able to move around to ensure its comfort, avoiding becoming too hot or too cold.
- When you transport your dog make sure they are comfortable and safe at all times.