The Lucas rotary headlamp switch from 1970s Triumph Dolomite / 2000 / Stag etc can be made to fit and will even take the original knob. The angular positions for off / side / headlamp are even the same as the spacings for the engraving on the brass backplate. It also has a very positive click.
If you are not a stickler for originality the Lucas ignition switch that Tony shows on his dash board can be converted easilly to fit the M Type back plate. All that's necessary is to throw away the large securing nut and cut a few threads off the body so the barrel pokes through the original ignition switch hole in the brass panel. The bakelite casing for the new switch fits directly over the original threaded pillars. The switches are easilly available at autojumbles for a couple of pounds and they have a proper key. Best to look out for one which is simply off/on without the third, sprung position for the starter.