Following issues with using data in Portugal a few years ago with excessive data usage on my iPhone, I did wonder if I should follow the same process I had used in October 2023 when I went to Amsterdam, last year when I went to Europe, and when I went to Helsinki in February.
The process I would use is go into Settings -> Mobile Data and turn off virtually all the apps which could use mobile data. I would turn off mobile data for virtually all my apps and also specifically iCloud Drive and iCloud Backup. I am pretty sure they were the culprits for my excessive data usage whilst in Portugal. This time I only turned off mobile data for iCloud Drive and iCloud Backup.
This trip was a six day trip to France, with a one day excursion to Bruges in Belgium.
However on my final day I ran out of data, having used the 12GB of fair use data. I had to spend £6 adding a single day of unlimited roaming data. Always a challenge to work out what app or process was the culprit, but I think it may have been Photos and iCloud. I also used the phone extensively for CarPlay.
On the side trip to Belgium, my phone did automatically switch to the Belgium mobile network, and switched back when we returned to France.
One interesting thing that did happen when we visited Cap Gris-Nez which is the bit of France closest to the UK was that my phone did switch to the UK network before switching back to a French network when we moved away from the cliff edge.