As of the 1st of October 2024 it is a legal requirement to register poultry and other captive birds with APHA. The UK bird-keeping world has been broadly divided in to those who have 50 or more birds and those that have 49 or less. There are exclusions for small numbers of pet birds that live the house and these are detailed.
See Poultry and other captive birds: registration rules and forms - GOV.UK (www.gov.uk) for more information and to download registration forms.
Note that at present their online application system seems to be down so keepers will need to complete the relevant downloaded forms and submit it via email.