French capital gains tax on UK property sale

[LOUIE]

Hi folks
We are trying to buy part of the garden of a UK property to build a house. The seller is in the process of moving to France (not clear if he is already resident there; he thinks not at the moment). He is considering withdrawing from the sale due his potential French tax liabilities if we cannot complete before he does become tax resident in France but in sufficient time for him to establish French residency before Brexit. Whilst we would hope to be able to exchange, legal problems means that if Brexit does happen in January we are very unlikely to have completed the sale. I understand completion rather than exchange is the tax trigger date in France - is that correct?
I am trying to get a handle on his situation if he does sell after becoming resident in France (there would be no UK tax as it would be covered by PRR relief).
He has owned the house and garden for about 25 years. The house itself is not being sold.
As far as I can see, he would not have to pay French CGT as he has owned the property for more than 22 years. He would have to pay social charges subject to taper relief of 46% or 55% depending on whether he has owned it for 24 or 25 years. I am not sure of the rate - would he have to pay at 7.5% or at 17.2%? I believe the supplementary tax on larger disposals would not apply as this is tapered over 22 years rather than the 30 years for social charges - correct? There is no annual exemption as in the UK so the whole gain would be charged - correct?
Anything else I need to know?
Any help would be appreciated.