"Where a single carriageway has four or more lanes, use only the lanes that signs or markings indicate."
This one has only three at that point: LH filter, the disputed one, and the one going the other way. Though TBH the second half of that quote makes sense regardless how many or few lanes.
"You should follow the signs and road markings and get into the lane as directed."
So I did what the highway code says, I got into the correct lane. But in practice, as I said in the OP and in my email to the cops, I appreciate it's irritating when someone cuts in on what you regard as "your" lane, especially if they've "cheated" by using an empty incorrect lane to get there, though technically the idea that you can "allow" someone or not into your lane is nonsense,
especially if you're stationary at the time. If someone's in front of you, they're simply in front of you. As cyclists we live with that all the time, people overtaking and then squishing back in right in front of us. It's annoying, but once they're in front of you, you just have to put up with it. What it's down to in the end is whether you have 2tons of motor at your disposal or 20lb of tubing - "might is right". What I agree I did specifically wrong was not to get RIGHT in front of the guy, but seeing what happened, do you really think he would have been mellow about it?
BTW the cop in question didn't take the opportunity to comment that I'd done anything particularly wrong, even though I did tell him I'd cut in from the LH lane.
I occasionally drive a large-ish vehicle, I wouldn't dream in a million years of using it to intimidate someone let alone shove them around. If someone can get past me without impeding my progress, I'm feckin delighted.