I am the driver of car B. It's just words, kept as simple as possible. Everyone is aware that the cars are passive and wouldn't be doing anything without the input of the driver. That comment is a little pedantic tbh.
But anyway, yes I was driving car B, I saw him reversing out, as I approached, I slowed in anticipation, he then paused the manoeuvre, being stationary on his side of the road, all being it a bit askew, but with enough room to pull forward onto his side of the road and drive away. As I got closer to the spot he then reversed back straight across the white line, still at an angle , into my carriageway, at fair speed for reverse and continued to do so as I braked, moving further into my carriageway, towards me, meaning I also had to start to swerve to avoid him had I not stopped in time, which I did, but it was close.
I was doing less than 15 at this point, from 20 (the limit) when I first saw him. There was enough room for him to have got onto his side of the road from the position he paused in, without crossing the white line at all. The road is wide enough and he had enough space to do so.
Had I not been aware, slowed down or paying attention then I would have hit him, had he not paused in the manoeuvre I would have stopped completely earlier however it appeared that as he paused and had enough room to drive straight off on his side of the road he'd finished reversing, and I couldn't see his reversing lights from the angle.
I clearly misread his intention, although I can't think why he would have paused, waited for me to get closer, then reversed quite fast straight into my path when he had enough space to pull straight off onto his side of the road. He wheel spun off onto his side of the road when I'd stopped, and continued at speed along the road.
Maybe I should have stopped completely, even though he did, but until he actually shot back there was no reason to as he wasn't in my path at that point, I slowed in anticipation but didn't stop as he did, and was mindful of the traffic behind me.