I really try to avoid threads like this as it seems my opinion is not in the majority. But I'm drawn inexorably like a moth to a flame......
FWIW, here's what I think. Forget about what the Highway Code says for a minute, I'm just considering the reality of how car drivers drive.
When I'm cycling along on my own I get passed by cars. Practically none of them overtake me in the way they would another car. What they actually do is give me (in my opinion) enough room and that leads to them straddling the white line a bit. Some more than others. I'm happy with that, doesn't feel too close to me. Presumably they feel happy with that too. What this does do on quite a lot of roads is still give enough room for a car coming the other way at the same time.
Now of course some folk will say that's not safe. But that's the reality. When I'm driving I see this all the time; car coming the other way overtakes cyclist, straddling the white line. I see him and make sure he and I have enough room to continue driving safely.
Fast forward to three abreast. The driver wanting to overtake can't do the above. By overtaking he feels that he has compromised the space on the other side of the road to the extent that a car coming the other way hasn't got enough space. So he doesn't overtake
Of course I understand the argument that a car driver should give as much room to a cyclist in overtaking as they would any other vehicle. But that just isn't the reality and that's from where this difference of opinion is derived