As you are resident in the UK, HMRC has the right to tax your world-wide income. This includes your income from France.
You would need to complete a Self Assessment Tax return and declare the income in SA102. If tax was deducted from the foreign employment, will also need to show the income and tax deducted in SA106. This will allow you to claim a credit for the tax deducted.
You can register for self assessment at:
Register for Self Assessment
All boxes in a tax return are in pounds sterling, so you may need to convert your income and tax using the official rate which can be found at:
Exchange rates from HMRC in CSV and XML format