Andy, i paid under £10 for the used Fiamm horns on ebay delivered. I did have a look at new ones but they looked low quality. I would guess Jaguar expected quality ones when they ordered.
Only thing to consider is these horns are high in wattage so adding a small relay takes the load off the horn switch and tiny wiring used in modern vehicles. The Jaguar horns also came with a large mounting bracket which the horns were fitted too.