If you volunteer any information regarding the number of poultry you keep it is your own choice. No one can make you give them that / or any other info if you choose not to. Unless your chickens are running around killing or causing grevious bodily harm to people.
Voluntary registration – less than 50 birds
You can register 49 birds or less, including any you keep as pets. The Animal and Plant Health Agency (APHA) encourages you to register even though you do not have to by law.
This is not a law. It is a "requirement" / a "rule", meaning you have a choice to play along or reject such nonesense. The bodies only get away with these info-gathering measures if people volunteer their info.
Key word: New registration requirements = requires / needs you to take part.