@geoff_faulkner I have recent experience and can tell you (to a point!)
As far as the UK is concerned, you need to pay CGT there and it is not a terribly straightforward calculation. It’s based on the increase in the value of the house since 2015, combined with the amount of time you’ve been non-resident less 18 months! If you were, for example, resident in France for the whole of that time, you would have to calculate it based on the difference between what the house was worth in 2015 and what you sold it for, but the period of non-residence would be from 2015 to the time at which you sold it. I strongly suggest you actually talk to HMRC about that - they have become more responsive to phone calls recently.
As far as France is concerned, nominally your tax is based on how long you have owned the house for, More than 30 years and you can forget tax. Lower than that and some tax needs to be paid. I contacted the local Tax office to talk about this and they said I didn’t need to pay anything because I’d paid in the UK. This is in direct contravention to the published rules on the tax website here so I have kept the emails!
Unfortunately the dual taxation agreement only applies to income, not capital gains so it is very possible you will end up paying tax twice. Sorry about that but if your local tax office here is approachable, please go and talk to them!