Since the UK has ratified the International Convention on Salvage, 1989, is it necessary for the master of a UK ship to establish agency of necessity before making a salvage agreement?

No, because the Salvage Convention provides: 

(1) that the master shall have the authority to conclude contracts for salvage operations on behalf of the owner of the vessel; and (2) the master or the owner of the vessel shall have the authority to conclude such contracts on behalf of the owner of the property on board the vessel.