It is easy enough to demonstrate no VAT is due (especially, I think, with the very old 1980s and older imports) but that is not the same thing as having the CofFC which the French (and others) are allegedly looking for.
And under EU law, the Customs must deliver a CofFC upon request. The UK Customs have been acting illegally for the last few years.
I know for a fact, from the number of people who contact me, that a lot of people out there haven't got that magic paper.
IMHO, most pilots who don't read the forums (which must be most pilots) won't even know about this.
It may be the Frog Customs are happy with a purchase invoice... who knows?