AFAIK there has been no increase in French tax 2021 for 2020 income. Without more detailed information, it is impossible to be helpful other than to ask if you correctly filled in the tax return as respects pension income and if you are resident in France, why are you paying UK tax at all unless it is in respect of UK Government Service pensions covered by the Double Taxation Treaty.
If you believe the assessment to be wrong, you still have an opportinity before the 15th Dec 2021 to correct your return. Have a look through the Tax 2021 topic