As a Uk resident, HMRC has the right to tax your world-wide income and capital gains and this is declared in a Self Assessment Tax Return. Even if the gain from the French assets disposal is tax free in France, it is still a taxable gain in the UK. You would need to use UK capital gains rules to work out the gain. You would need to workout the acquisition costs in pounds sterling using an exchange rate in use at the time. You would do the same for the disposal and disposal costs, using the exchange rate in use at the time.
Under the terms of Self Assessment, we do not provide an official exchange rate and the onus is on the individual to use a just and reasonable exchange rate for each acquisition and disposal.
You may wish to use the exchange rates from the tables at these links or from another source.
Exchange rates from HMRC in CSV and XML format
Foreign exchange rates and spot rates: 1 January 1989 to 31 March 2009
There is a capital gains calculator below to help you work out the gain.
Tax when you sell property