Options for adding a timer on heated towel rail socket in UK toilet
Allowed, but there are lots of timeswitched fused connection units available if you'd prefer not to have a plug/socket.
Selection here:
https://www.tlc-direct.co.uk/Main_Index/Timers_Index/Immersion_Heater_Timers_1/index.html
Edit to add: all sockets must now be RCD protected. If the existing circuit is not RCD protected you must not change the fused connection unit to a socket.