When i was rebuilding my moggie pickup i acquired a pair of air horns & compressor from an Italian fire engine, i had to remove the alternating tone gears from the works & just used the deepest one of the two horns. It was quite loud!
XJ6 uses a pair of high & low tone Lucas horns iirc. A town and country selector switch was common fitment on expensive cars, air horns for country and two tone for town.