O2 is decent but has the same total offing useless call centres, who say all the right things but do not do any of what was promised.
However I have been with o2 for almost 20 years, from the time they were BT Cellnet. Why? because where I live, they are just better than any other. And I check annually.
Now this is where the problem starts: Giff Gaff is on the O2 network.
So you will have Wifi calling at home, but as soon as you're out of wifi range you will have the same low signal as you had with tiff gaff