British metal boxes aka backboxes can be set in the wall way below the plaster line, it doesnt really matter. it isnt uncommon to see them 1" below the plaster line, you just use longer screws! This is because our socket and lightswitch fronts are only in one piece so the screws clamp the accessory to the plaster.

I will try to find a picture of one installed somewhere.

I took my time, I hurried up, The choice was mine, I didn't think enough