Remote Switch For Fused Outlet

[JohnW2]

There is already one 2.5mm cable running up to what is/was a single spur socket. Was hoping to use that as my switch wire for the lower socket without having to crack into the wall and install another cable.

Ah, you didn't mention that! In that case, as others have said (although it still feels 'orrible to me!), although it's very unconventional (and therefore may confuse future electricians), I can think of no compelling reason why you cannot do as you proposed, in the name of 'convenience'. It is, as you said in your OP, no different from light switching.

One question - do I take it that you definitely want/need this switch? It's not that unusual for appliances to be plugged into unswitched sockets, with no upstream switch/FCU.

Kind Regards, John