No consent to which you refer is very unlikely to be in any clause relating to user, just alterations. While alterations are inevitably involved, the leases restriction what you can use it as, no matter the planning consent you might obtain or might not need.
As to the premium you must understand that you and the LL entered into a lease defining a particular use and he is under no obligation to agree.
To the extent that they may, or the lease as suggested is so broad or ill defined that it may not preclude such use, or does so subject to consent, as well as requiring consent for the alterations, there is a control that the landlords can seek a reasonable premium to reflect any damage to their reversion.
back to basics what does your lease say about what your premises are and are there any reference to use as JB asks?
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com