Regardless of what the Highway Code says, I wouldn’t be cyclng that quickly down the left of slow / stationary traffic, where’s there’s junctions on the left that traffic coming the other way might be turning right into.
Who’s relatives are going to be comforted to know that their loved one’s last words were “but I had every right to do that, the Highway Code says so?”
Very rare that I filter down the left, very often I’m riding past traffic, and I generally do so on the right. Not had any collisions. Had a few potential near misses, but since I’m on the right I can see them coming.