I suspect that the drain due to network really is that, it's due to network issues and not the device itself.

For example my Jolla sbj-1 charge is 83% now, this means it is 17% down from full charge it was on 7:00 this morning, that's 10 hours ago which yields about 1,7% drop in an hour, right?

And this is the original battery I got with my "First One" device about 3,5 years ago

I have made 2 short calls, read emails few times and did very light websurfing checking some series stats during the day.
I drove about 50km to a floorball match and back home again, so the phone has roamed a fair lot. As far as I know I was on LTE network all the time.

Conclusion; you have really bad network coverage in UK and your network planning really sucks.