Is it possible to get a certificate of breed for an adult dog in the UK?

There is no certificate that would overrule a decision made in France, any more than a certificate issued by a vet or show judge in another country would overrule UK legislation if the dog looks like a Pitbull.

If your dog is seized you would have to prove he is not a Pit, and you cant. Dont risk it.

If this is just a holiday, if you are at risk of having him seized where you go, quite honestly I'd leave him at home, either with a friend or relative as a sitter, or an insured sitter of in a good boarding kennel. It gets hot on the continent in summer, so he may well be far better staying at home - and you will be more free to do what you want to do, go where you want to go etc. in France.

http://www.pet-owners.co.uk/Expert-Advice/Legal-Ad...

I did a quick search (there are a number of websites other than the link here) and found this ..... some of it may help?

Honestly, since this is just for holiday, you'd be better off to have him boarded at home or leave him with a trusted friend or family member. If you took him, you'd risk him getting seized. It would takes MONTHS to get your dog back, if you could get him back at all.