Obviously, I can not comment on what the landlord will do but you could take legal action to prevent anything unreasonable. I would look at drafting the variation of the lease and having a lawyer send it to the landlord simply for their approval then you can register it with the registry. If you cannot agree to this, you could look at taking the matter to court an d the court can look at this matter, if they deem it reasonable, you can have them approve this change without the landlord's consent.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
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