No direct experience, but your way through this, I would suggest, is as follows:
- An “allotment garden” means an "allotment not exceeding forty poles in extent which is wholly or mainly cultivated by the occupier for the production of vegetable or fruit crops for consumption by himself or his family"; and “agriculture” includes foresty, horticulture, or the keeping and breeding of livestock" - s.22 Allotments Act 1922
- the keeping of livestock is therefore not included in the definition of an allotment garden
- s.10 Allotments Act 1950 allows hens and rabbits to be kept on any land even if a tenancy, lease, contract or any other document says they can't. This would include any allotment leases
- there are no similar provisions relating to pigs
- therefore, neither the definition of an allotment garden, nor the later rules about hens or rabbits, provide a basis for claiming that pigs can be kept as of right on an allotment garden. The fact that the legislation only specifically refers to hens and rabbits, provides a strong argument that it does not extend to any other livestock such as pigs (if Parliament had intended it to do so, it would have said so)
This all assumes that your allotments are indeed allotment gardens. If it's private land being used for non-statutory allotments, then it's up to the landowner to decide.
PS. You also mention the fact that houses surround the site. Allotments are not exempt from the requirement not to cause a statutory nuisance (noise, smells). I'd be surprised if pigs never risk crossing the nuisance line.
PPS. I'm not familiar with any planning regulations, although they may exist.
But I have done a quick search and would have concerns around things such as whether an allotment would be large enough for the pig to safely be kept and housed long-term (compliance with the Animal Welfare Act 2006), the rules around licensing and registration of the pigs, rules around transporting them (generally and in appropriate vehicles), and the infrastructure that might be needed on site (would it require significantly reinforced fencing, for example, to prevent them getting out - and what impact might such infrastructure have on neighbouring plotholders).
What does your council say about local zoning rules?
Does your council have an environmental health team who might advise?
This link appears to be fairly balanced about issues to consider.
