Hello, I'm Keith and happy to help you with your question.
This will be an employee share scheme not approved by HMRC. You would be liable to Capital Gains Tax (CGT) in the UK on the gain made on the sale. However, that gain is based on the price at vesting and in most of such schemes participants sell on vesting so there is no capital gain to tax. If there is a gain to be taxed then you can deduct your Annual exempt Amount of 11K and any surplus is taxed at 18% or 28% or a combination of the two rates depending on your income including the gain in the year of sale.
You will also have to think about the French taxation possibilities. French tax is so complex and so constantly changing that you are always advised to employ a local professional to guide you through the maze; indeed sometimes French Law makes this compulsory. If is far from simple as you will see if you have a look here:
which is enough to make your head spin. The only consolation is that under the Double Taxation Convention between the UK and France you can only be taxed in one jurisdiction and this extends, as far as the UK is concerned, to CGT.
That is a very brief resume of your situation and a lot depends upon the detail, the devil is always in the detail. I do hope I have given you some food for thought with my answer.
" However, that gain is based on the price at vesting and in most of such schemes participants sell on vesting so there is no capital gain to tax."
What does the term 'Vesting' mean and why does it have an impact
The value of 100 shares at current prices and euro exchange rate is about £3600