Assuming you know exactly where you want all the sockets...

Thanks for all the input.

This morning I made up a template in thin ply, single and double
patress box cutouts, 450mm and 1100mm off the floor. It's designed to
stand on the floor so that the square holes in the board are in
exactly the right place for sockets and switches.

Cut plasterboard to size, slap template against board as it's stood
where it will go, cut holes with multimaster, slip boxes through
holes, and screw to studs, lean board forward and push cables through
patress boxes and through holes in board, screw board to studs.

Speed - pretty good (I'll get faster). Results - very neat.

Upsides - absolutely consistent positioning of sockets and switches,
and measuring errors eliminated. Downsides are it's a bit fiddly when
there's several sockets on the same board.