First stop for me would be HM Customs and Excise free telephone service. Someone there should be able to help.
Only other thing I can suggest is a "deposition" signed by a Public Notary to the effect that you have woned the boat for XX years and paid the VAT applicable at the time. French are very big on "Notarie Publiques" so could suffice but bad news is "Have you seen how much they charge?!!"
have a similar problem - indeed many people have, since the only document that is acceptable beyond doubt is the original vat invoice and many people simply didnt keep them because there was then no reason to do so.
what you need is a body of evidence that vat was paid in the first place, and that the boat has been kept in the uk since that time (ie that it has not been exported and the vat reclaimed). marina/ boatyards receipts over a period of time etc.
accumulate all this documentation, and then write to customs asking for a letter stating that they are happy vat is not due. they will oblige if the evidence is convincing.
then all you have to do is to persuade some foreign customs man to accept the decision of a british one. should be no problem since vat on a transaction between 2 british citizens occurring in the uk is a matter for the uk govt. not the french, spanish etc. we shall see.
have more detailed info if you e mail me direct.
Seriously, I cruise France most seasons and have never, ever, been asked for anything to do with VAT. Passports yes (not often), registration docs and VHF licence yes, certificates of competance - never.
I suspect the only people who might take an interest are our own excisemen on return.
Ditto me - on the other hand I would hate to find out the hard way.
I always consider HMC&E an organisation not to mess with and in last years French fuel strike the French version demonstrated the same sense of humour I expect from our very own Customs people.
They caught a man driving a car with red diesel in his tank. "Pas problem Monsieur. Votre voiture avez les kilometres 120,000 en la cloche - s'il vous plais donnez mois Ff 120,000 pour le duane. Aussi s'il vous plait donnez mois une autre Ff 5,000 pour le fine c'est interdit vous avez the gasoil rouge en une voiture. Merci. Aurevoir. Avez vous un bon jour!!"
In case your wondering - my spoken French is even worse but I get by after a few whiskies ;O)
I heard of some officials on an island in the med. who wanted to 'confiscate' everything the owner didn't have receipts for as 'contraband' i also understand the british consulate was very little help. in the end they had to pay a large fine (aka bribe) to get away.