as an ex employee of www.emachcnc.com we used to use m3 to turn on spindle and m5 to turn it off, the output from the controller switched a relay, for added safety we supplied the inverter via a contactor, whose coil was switched via a key switch, as far as the HSE were concerned, as long as the facility to remove a tool ( key ) to make safe the spindle, it was deemed safe to handle.