A premium (or key money) is a sum of money charged by the previous tenant, payable by the new tenant for the right to take over an empty shop for the remainder of the lease.You get nothing more for your money than that. Premiums were routinely charged in the late 1980s when the economy was booming but fortunately, this practice does not seem to have been resurrected since and I am not aware that premiums are currently being charged.
Another practice that was commonplace in the boom-time of the late 1980s was the need for prospective tenants to make payments to property agents just to ensure that they were sent details of properties that were available to let. Those who were not in the know about this did not receive any mailings. I do not believe in subscribing to such practices but suspect that some agents (who have not sent me information on shops I know to be available) still operate in this way.
