Article 6 of the UK/France double tax treaty here deals with capital gains and Article 24 deals with a situation where tax is due in both countries.
I've found some notes on French CGT which appear to be up to date apart from the fact that non-UK residents can be liable to UK CGT as I explained earlier, from 6 April 2015. However, any CGT you pay in France will be deductible from the UK CGT liability if there is one. The notes start here.The basic rate of CGT in France is 19%. There is also a social charge of 15.5%. Whilst the French CGT charge is deductible from a CGT liability in the UK but the social charge isn't as HMRC and the European Court of Justice agree that it isn't tax. There is also a supplementary tax based on the size of the gain. There is some commentary on the social charge here. I suspect that only non-French tax residents will qualify for exemption from the charge or reimbursement if they have already paid it.A tax resident of France will qualify for discounts from the gain arising from the disposal of a property outside France on the same basis as a property in France that is disposed of. There are discounts for the period of ownership of a main home as described here and the costs that you can claim in computing the gain are set out here.
Hi again.As I said in my original answer, there is much of contradictory information on French CGT on the internet mainly because of the French government changing the rules almost on a monthly basis. It seemed like that at one point at least.The link I gave you was a little vague on the French CGT situation for a property which has been your main home but has been let before you sell it or you move out before you sell it. It talked about a year after moving out to sell it and no allowance if it was let. What it didn't definitively say as you will read in the fourth paragraph here is that the loss of main residence relief applies to the whole gain, not just the period of ownership when you were not living there or it was let.As I advised earlier, this is a UK tax advice site and whilst I do advise people who are moving abroad it is usually to deal with their UK tax situation for the year they leave, the period while they are abroad and the amount of time they can spend in the UK whilst working abroad in order to maintain non-UK resident status and when they return, not the tax situation in the country they move to. I can only give you pointers and, given the state of flux tax has been in in France, the best advice will be obtained from a French tax expert.You don't have to pay for my answer if you don't feel it is worth what you have paid as a deposit. You can simply leave the answer unrated, ask for a refund and for the question to be closed. I'd prefer you to do that than rate my answer as poor since, frankly it isn't and this is a UK tax site not a French tax site. It's an honest view of the situation in France and, even as someone who has been working in tax for 36 years, I'd still seek advice from an expert on French tax given all that has gone on there in recent years around tax if I was thinking of moving there.