The Home Office has published Guidance on the way in which the Animals (Scientific Procedures) Act 1986 (ASPA) is administered and enforced. The Home Office also published a Code of Practice on the housing and care of animals bred, supplied or used for scientific purposes - https://www.gov.uk/guidance/animal-research-technical-advice#code-of-practice-for-the-care-and-accommodation-of-animals
The Code of Practice ensures that the design, construction and method of functioning of installations and equipment of licensed establishments, along with their staffing care and practices, allow procedures to be carried out as effectively as possible in order to obtain reliable results using the minimum number of animals and causing the minimum degree of pain, suffering, distress or lasting harm. Further technical advice notes are published on specific issues - https://www.gov.uk/guidance/animal-research-technical-advice
The cornerstone for the protection of animals used for experimental or other scientific procedures is a risk-based inspection and assessment programme undertaken by the Animals in Science Regulation Unit. As part of this risk-based programme, Home Office Inspectors undertake visits to all establishments licensed to breed or supply animals, or to carry out regulated procedures on animals under the ASPA in Great Britain, to inspect the welfare, health and environment of the animals at the facility.