You're missing the point, Dopey.
Irrespective of who's in the 'right' and 'wrong', you have to deal with the facts as they are at present and where the balance of power lies.
The council do not presently accept that there is anything wrong with the tenancy. They state that vegypete can't have chickens. If he has chickens against their specific position, they will take steps to terminate his tenancy if he doesn't give them up.
If they did terminate his tenancy, he couldn't just refuse to leave. He'd be a trespasser. And would also blot his copybook for ever getting a tenancy again.
He would have to complain first through the official routes in the council that they had acted unlawfully. Probably wouldn't get anywhere.
If he gets his MP or councillor involved, that's even more likely to get the council's back up.
Then he'd have to complain to the Local Government Ombudsman in the hope that they would decide in his favour and make recommendations to the council to change their policy - but there's no way to make the council act on their recommendations (though they rarely refuse to comply).
Or he could take them to court for an order forcing the council to grant him permission for chickens. Which he might not get if the council has got its act together and finds grounds for saying there would be a statutory nuisance. And would cost thousands.
Far better to try to resolve it amicably without risking the above, even if the law theoretically does stand on his side.