Unswitched sockets

I am writing to try to help with an answer. Here in USA we do not normally have switched sockets for AC power. But I do know it is far more common in England for AC power outlets to have switches right on the outlet cover.                  
socket UK = outlet USA                      
The first thing to mention. A ground gizmo on the socket is of no value unless it is grounded INSIDE the box in the wall. So the first thing to say is you would need to know if this is available in your wall at the socket 'box'.

Yes there is an earth connection inside the (room circuit)  wire loom which connects to earth 

Then do you have a turntable ARM with an earth ground on the arm wiring? (My understanding from the original post is the turntable AC mains PLUG has only a two prong plug?) The earth ground from a turntable is usually with the tonearm wiring. It should be grounded to the phono preamp/box the phono RCA plug into. Whether any further grounding is needed? not likely.
The worst problem would be a ground LOOP. which is TOO MANY GROUND PATHS. So.. IMO.. if you have no hum in the system now from ground problems. do not deliberately add them! So I say NO to the added socket ground, particularly if the main plug going to the socket is three prong (grounded to earth) If it is the Turntable plug only two prongs, The turntable AC mains does not need a ground either.

Ok perfect - so well explained - that makes complete sense now. 
Was just thinking of times before where I suddenly heard improvement where I hadn’t realised there was a deficiency and thought this might be one of
those issues. But system seems good - will report back if any different.