giff gaff is O2 and O2 shares a parent company with Vodafone (Virgin Media O2) and also shares masts. EE and Three probably share infrastructure too, particularly in rural areas.
At least for me, Three was definitely working (South West)
Vodafone & O2 / VM are separate entities I think. In fact, 3 is being merged into Vodafone. Regardless, hard to know how to have a good backup, maybe starlink ;)
At least for me, Three was definitely working (South West)