I was with O2 before Three, and it was bad that I had to switch. Then Three turned bad years later 😅 However, I heard O2 has improved.
As for the merger, if that means better coverage by using signal sources set up by both companies, I’m fine with it. I don’t have much hope things will improve.
I believe one of the problems is taking out Huawei equipment, which has affected coverage and usage of 5G. All the companies have stayed quiet in what is happening to replace it, yet no reduction in bills for a poorer service. The regulator seem somewhat useless. I went to France and Sweden recently and was really impressed with the quality of service there