Carlt is right 100%.
Fiat no longer have production and stock of the Coupe bonnets. Two stories I've heard are the panel company who produced them in Italy had gone bust, and/or the mould/panel press machine for the Coupe Bonnet is worn, so much so that it no longer produces a good bonnet...
Fiat UK will show they can sell at £500 less discounts, etc. But only when you go further into the purchase does it show "critical". Meaning the part is no longer available.
Coupeteile (in Germany) seem to be able to get hold of the occasional primered bonnet. Perhaps they have a few still lying around in Germany.
Mariusz at AutoDS is looking at producing Fibreglass, and/or carbo fibre. But the logistics are an issue with getting such a large part into the UK from Poland at a good shipment rate. The solution to this is for someone to be able to stock bonnets, say 10 at a time, so when needed in the UK they can be collected, or delivered at a cheaper price.
But, as Carlt said, Fiat will not be able to get a bonnet to you.