Under the France/UK double tax treaty pensions from Governmental service funds are only taxable in the UK. This applies to government service lump sums.
Although not directly taxable in France (i.e. you get a credit for the French tax), governmental service pensions (and lump sums) are usually taken into account to calculate the effective rate of tax on your other French taxable income. This may increase overall French income tax liability.
However, this could be avoided if your take your full available lump sum at once (provided the fund was uncrystallised), and opt for the French lump sum treatment. No tax will be due in France on the lump sum in this case (i.e. you get a credit for the French tax), and the lump sum will not be taken into account to calculate the effective rate of tax on your other income.