Others used to have chickens on local council allotments. I found this on allotmentssouthwest.org
'The two exceptions, at least for council run sites are rabbits and hens. The 1952 Allotments Act states that plot holders have a right to keep hens and rabbits on council owned allotment sites; and also have a right to put up or maintain any buildings reasonably necessary for that. Any terms in your lease or tenancy agreement, and into any covenant, forbidding the keeping of rabbits or hens on council owned site is void and cannot be enforced by law. You must of course abide by all other laws such as health and safety and animal welfare. Councils do sometimes think that they have the right to tell plot holders what to do just because they are the council. This is not so as the Unfair Terms in Consumer Contracts regulations (1999) apply. These make any purely arbitrary, unfair council rules unenforceable. This is reinforced by the Human Rights Act, as well as the administrative court requirement that rules should be those of a free and democratic society.'