Landlord’s consent is usually given by a formal deed (known as a licence to alter) requiring the tenant to:
comply with various building requirements when carrying out the works
insure the works
remove the works when the lease ends.
The document will need to be negotiated if you do not want to remove all or part of the works at the end of the term. It should also specify whether the works should be taken into account on rent review (this should not normally be the case where the tenant has paid for the works).