Below is Section 9 of the Animal Welfare Act 2006. It falls under criminal law & therefore is directly enforceable by the police and Trading Standards (click here for the full Act):
Duty of person responsible for animal to ensure welfare
(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.
(3) The circumstances to which it is relevant to have regard when applying subsection (1) include, in particular:
(a) any lawful purpose for which the animal is kept, and
(b) any lawful activity undertaken in relation to the animal.
(4) Nothing in this section applies to the destruction of an animal in an appropriate and humane manner.