Adding a Contract Holder: Landlord’s Right to Refuse?

We have a situation where a Contract Holder has withdrawn from the contract, which started in May on a periodic basis, and the remaining CH has requested to add a new CH to the contract to replace the outgoing CH with immediate effect. The landlord has refused this as he intends to refurbish the property and plans on serving six months' notice starting in November, so he does not want to extend any contract dates by adding another CH.

Is it correct that the landlord effectively commits to another six months before they can serve notice when adding a contract holder to an existing contract?

Is there any legislation to support the landlord's right to reject the request for this reason? What I can find suggests the CH has a legal right to request it, and the landlord must give it due consideration, only refusing the request if reasonable.