I think it was the Triumph Herald which had a 'reserve' cock on the fuel tank. Problem was, it was in the boot. So not much use in an aeroplane! Personally, I've no idea whether the 'low state' fuel caption works in my Prelude as I've always diverted to the nearest fuel station when I'm down to 1 segment on the LED fuel gauge.....and I apply the same logic in aeroplanes.
Surely there must be someone out there who can design an accurate, reliable fuel gauging system for light aeroplanes? The best one for a PA28 is a calibrated piece of wood to dip the tanks - the manufacturer's gauges are utterly useless.