If you have a plug-in pocket socket which is networked, then you don’t need to do anything more with the wiring to be able to control it with a separate switch. That’s one of the advantages of SmartThings. The control switch will talk to the hub, and the hub will talk to the pocket socket and tell it to turn itself off. The control switch does not have to be on the same circuit as the pocket socket; in fact it can be battery-operated. It just has to be able to talk to SmartThings.
The only issue here is that if your home automation system goes down, but your power is still on, then the switch will not work. But presumably that’s a temporary situation, and many of the pocket sockets have a button on them which allows them to be turned off.
So if you assume that you’ll handle emergency downtime in some other fashion, I would think the battery-operated pop switches which @adamv has created a device handler for, would be a good choice for most UK homes.
Again because the way these are used with SmartThings is to have the switch talk to the hub and then the hub talk to the pocket socket, you’d be able to use them to control either a zigbee pocket socket or a zwave pocket socket. Or a group controlling multiple devices of multiple protocols, such as a couple of lights around the room plus a fan plus even Devices in other rooms as well.
Here is a link to Adam’s topic:
And this is one of the devices, but there are a number of choices that can use the same device handler. 