The two wires you have in your existing UK wall socket are the live feed in, and the switched feed which goes to the lamp. In a UK wall plate the neutral is NOT present - in fact it is not allowed to be due to the risk of a short. You therefore have no neutral available to power a wall switch assuming it needs mains power, so probably cannot use it.
I disagree Steve. Yes it's true that you will typically have only two conductors in most one way switches in the UK. It is perfectly acceptable, however, to have a neutral running to the switch, providing it has been properly terminated (dead end terminated if necessary). When quoting for a re-wire, I offer a service in which I make switches ready for smart home features. I run three core cable (plus earth) to the switches which provide conductors for all four (Line, Switched Line, Neutral & Earth). This only costs an extra £40 or so in cable with no additional labour charge. Also, on ALL rewires, I install deep 35mm back boxes on all switches and sockets to allow for the increased depth of some smart/decorative wall plates (such as the Sonoff Touch). Of course, this service often leads to follow up work installing smart home systems.
I admit I may be out of date on 17th Regs - but I would suggest that the percentage of home wired like this must be absolutely tiny sadly.