It now appears that Three could be the only operator with enough 5G coverage for the iPhone 12.
Of course it's going to depend where you live, but in my experience this is nonsense. I have a Huawei 5G WiFi router and have used it with both Vodafone and Three SIM cards in London.
The Three 5G network is currently rubbish in London, despite their flashy marketing. Very limited and spotty coverage, and if you don't get 5G (which is most of the time) you will drop to their absolutely terrible slow 4G network.
Vodafone's 5G coverage on the other hand has improved hugely during 2020, and now I seem to get a decent 5G signal through most parts of Central and East London where I go. And if you don't get 5G, you drop to Vodafone's great 4G network which is usually really fast anyway.
Speeds on Vodafone 5G are brilliant, I usually get 300-400 Mbps at my flat, so it's replaced wired broadband for me. I'm just hoping that the launch of the 5G iPhone doesn't affect this too much!