As title, would like to know please? Thanks. Peter.
Whether or not it can be done, I cannot recommend it. By putting a 16A fuse in a UK domestic plug you are potentially overloading the cable (max for a UK domestic plug is 15A rated according to this pat testing website).
A spur off a ring main, using 2.0mm2 twin and earth cable is rated for 20A. Two 16A plugs into the sockets on that spur could hence significantly overload it, starting a fire in your walls.
No. It's illegal.
Any device that isn't intended to be fitted with a plug, cannot be fitted with a plug.
Any fuses must comply with BS 1362, which allows for a max of 13A.
Your device (Immersion heater?) must be connected to a suitable circuit by a fused connection unit (FCU) - just adding a 16A load to an existing circuit would be a terrible idea.