Seems to me that sometimes these admin people simply make up the rules as they go along. If the lady you spoke to knew her job properly, then she would already know that Government Pensions are always taxed in the country of origin rather than in the country of residence of the recipient.
I have been declaring my Government Pension on my French tax return for 16 years, and have never been asked for any documentation of any sort. Technically speaking, the UK P60 is in an official language of the EU (English), and should therefore be accepted as it is (without translation), but often that is not a fight worth the trouble of pursuing.
If you give them a copy of the P60, together with a conversion of the figures into Euros by multiplying the figures by 1.1225 (the official conversion rate for this years tax returns), that should suffice. You could also give them a simple handwritten Attestation sur l’Honneur to the effect that the Government Pension has already been taxed in the UK as is evidenced by the P60 forms. Having your signature witnessed by the secretary at your local Mairie will also add weight to your evidence.
To counter the problem of the difference between the UK and French fiscal years, the system I use is to add one quarter of the figures from the previous year’s P60 (2017 - 2018) to three quarters of the current year’s P60 figures (2018 - 2019), and by so doing one has a justifiable figure for the calendar year of 2018. I do these simple calculations in £s, and then convert the resultant answer into Euros.
The response I always use is to ‘Drown them in Paper’ whenever officialdom gets pernicketty, and over the years I’ve found that it usually works just fine.
Good luck and keep smiling.