I’m guessing Flora Blanche was a shipper’s brand that vanished when J R Parkington stopped trading in 1996. I also doubt it was a real Sauternes. This was before UK joined EEC/EU and accepted wine laws so UK bottlers could call their wines anything.
The description of it being a light Table Wine that’s not too sweet doesn’t sound like what we know today of Sauternes.
My parents drank a Spanish Sauternes with dinner, so it couldn’t have been as sweet as real Sauternes.
Yes, it’s possible to buy ancient bottles of Flora Blanche at auction but they’ll be 40+ years old, and even if drinkable, will not be at all like the wine your FiL drank back then as he wasn’t drinking 40 yo wines.
If you do get a historic bottle of Flora Blanche I suggest also getting backups, first a half of Sauternes, cheap ones are available from supermarkets. Your FiL will probably find it too sweet so also get a medium sweet wine such as Cour-Cheverny Moelleux, Cuvée Renaissance, Le Petit Chambord, Cazin 2018 (thewinesociety.com) which will probably be closer to what he enjoyed then.
But not only wines have changed, so has his tastebuds. Who us now drink what we did 40-60 years ago? (Those not born then are exempt from this rhetorical question)
(Much later edit - of course as UK joined the then EEC in 1972 the company folded after - not before - however I suspect that without being able to sell their bestselling brands, of which Flora Blanche was just one - they could not compete in the new market)