DR House, My idea of putting the cable in conduit is to provide some protection
to the cable in case someone, sometime decides to hang something there-key hook,
painting,whatever- power wires are supposed to run through the middle of the studs for this reason.
Your idea of using thicker plasterboard would not work because of the architrave which is to go
around the door frame.The next soloution is to put the switch on a mounting block,which then doesn't
need any space behind it.If the cable in conduit is not permissible by its self because of the above
reasons,then you could look at other options such as putting some flat 2-3mm steel sheet over
the top of the conduit as well,and use a thinner plasterboard-I think you can get a 6mm thick one
which I have used for bending around curved bulkheads.