You haven't given enough information, like how the outbuilding is supplied and if it has a consumer unit or distribution board, and what sort of earthing your system is.
I would install a dedicated circuit with 16A MCB protection. I would make the connection to the kiln using an unfused double pole switch connection unit, wired direct with no plug+socket.
I would not RCD protect a Kiln. The element is likely to leak anyway which could cause tripping when first switched on after a period of non-use, but a non-portable appliance of this type does not present a significant electric shock hazard if the installation's earthing is correctly installed. However, this means it should not be connected via a socket outlet in an outbuilding as the socket could easily be used to power a portable appliance.
The circuit might need RCD protection against a high earth fault loop impedance. This would require details of your earthing system, and how the outbuilding is connected to the supply (including cable type and length) in order to determine if such RCD protection is required.