One of the few duties of a parish council is to provide allotments (where it is possible to do so) if requested by, from memory, six electors so yes its OK. My own council doesn't run allotments but those that do tell me that they can be a bit of a nightmare to manage! The fact that only a few benefit at any one time is irrelevant as properly managed, any elector could benefit if/when an allotment becomes vacant and someone else can take it on.