When in my car behind cyclists on country roads I
Not even if someone is reversing towards you? Seems entirely appropriate.
I had a large pickup reverse all over my bonnet at a set of on demand lights. He thought the lights were taking too long so backed up intending to trigger the sensor again. Unfortunately he could not back up far enough due to me being there. At pulling off my now crumpled bumper he jumped out to see what he'd done. Just as the lights changed as per the on demand cycle. They'd detected his presence it was just at the wrong time in the lights cycle to change for him quickly.
Anyway, the young lad was in a right state of panic when he saw me getting out. I really had to calm him down until his pregnant wife turned up in a car driven by his mother. They took over, mother calming him down and wife telling me they were going to accept full responsibility and have the insurance documents at their house on x street. I followed them and got the details, address and name. Insurance fixed the car in a fortnight and I had a nice car in the meantime.
However I did find out hard to use my horn what with getting into reverse and starting to move back asap to lessen damage. I think by the time I was moving and applied do the horn he was practically on top of my bonnet. It still worked as his pickup bed was not much lower down than the top of my bonnet. If I hadn't moved back a little bit and he stopped on hearing my horn he have hit the windscreen.
So as far as I'm concerned in that situation I will use the horn. Rules or no rules!