Then they wouldn't be two private individuals!
That's true
but how do you know that the chap who sold it to the chap who sold it to you paid the VAT on it?
Passing on the plane doesn't cancel out the obligation to pay the import VAT. If it did, there would be a trivial workaround for the Cert of Free Circulation / C88 or whatever.
I don't want to sound alarmist, and owners of ancient G-regs (which have been in the UK for many years) who pop across to France etc, should have nothing to fear because of the 1980s amnesty, but anybody buying a newer plane needs to get this sorted before handing over the money.
It's no different to buying a plane with no engine logs - you have to discount the price by the cost of an engine overhaul.