I was flying off to Finland and would be using my iPhone out there.
Following issues with using data in Portugal a few years ago with excessive data usage on my iPhone, I followed the same process I had used in October 2023 when I went to Amsterdam and last year when I went to Europe.
When I was on the plane at Heathrow, I went into Settings -> Mobile Data and turned off virtually all the apps which could use mobile data. Took me a while as there were a fair few. I turned off mobile data for virtually all my apps and also specifically iCloud Driveand iCloud Backup. I am pretty sure they were the culprits for my excessive data usage whilst in Portugal.
As and when I needed to use apps, I either used the hotel wifi, or switched on mobile data for specific apps.
I didn’t run out of data this visit, it was a shorter visit, just a few days. All worked fine.
I was also impressed with the mobile data speeds in Finland, which to be honest knowing their history (you know Nokia) wasn’t too surprising.