@larkswood12 has previously (helpfully) posted about a practical exchange rate methodology proposed by tax inspectors. You take the Banque de France (BdF) exchange rate at or around the previous calendar year end, and the BdF exchange rate at or around the actual tax year end, then average the two.
The rate at 31.12.23 was £1:€1.1506
The rate at 31.12.24 was £1:€1.2049
That would give an average of €1.17775 for calendar 2024 for regular recurring income. For one offs like pension lump sums, sale of properties, the spot (or actual) rate should be used.
Obviously there are a variety of different methods available, some proposed by individual tax offices, others by magazines etc.
The difference between the rates at the beginning and end of the year are startling!