Actually beeping the horn to show your annoyance is contrary to Rule 112 of the Highway Code.
As I'm an ex lorry driver, let's revisit the scene and see it from my perspective..
It's a single track road; not a single lane road. So there is nowhere to stop or pass other vehicles. It may be derestricted and subject to the NSL, but I'll wager that the visibility and the general road conditions do not warrant driving safely along at that speed. Even so, the OP becomes distracted by the phone falling onto the floor.
Driver distraction is a big cause of RTCs, so the OP does exactly what she is supposed to do: She waits until the road opens out so that it is wide enough for traffic to pass unimpeded, then indicates left and moves over as far as possible and stops.
She has done nothing stupid or illegal. In fact if she's distracted by the phone on the floor, the best and safest thing to do is to stop and recover and sort it out which is what she does.
I'm sitting much higher up in my cab and have a much better view of the road ahead, so pull out to pass the stationary vehicle, and to warn the driver I'm passing, I sound my horn. I might also flash my lights so she catches the 'blink' in her mirrors.
Judging by some of the comments in this thread I'm questioning how many have actually passed a driving test, or have any comprehension of hazard perception?
Here's a question for everyone here. How long have you been driving and when did you last look at a current copy of the Highway Code?
No need to reply, just go away and think about it truthfully, in the privacy of your own home