I think there's a "correct" way and a de facto way. I know over here in the States, a slowing down hand signal is the same: arm extended, palm flat down, slow up and down movement.....
Do I use it? Heck no. I'd guess 95% of people don't have a clue what it means. Likewise, the signal to turn right (remember, driving on the right here) is to have your left arm in a right angle turned up. I'd probably even venture 99% of people over here have no clue whatsoever what
that means!
So what do I use? To turn left, I extend my left arm to the left to turn left, to turn right, I extend my right arm to the right to turn right. If I'm slowing down, I put my right arm out a bit, with my elbow bent slightly so my forearm is approx. parallel to my side, with the flat of my palm facing backwards.
If I have someone drafting me, then yes, I'd just call out, or try to slow down gradually until they get the message. One time I did really offend someone who was drafting me (didn't know him, he just latched on). I breathed in a big insect, I coughed considerably, tried to call out a warning to him and then turned my head to spit out the offending insect as far away from him as possible. He wasn't very happy -but as I was choking, what was I supposed to do? Personally, I think you draft at your own peril and risk -it's your choice -after all, you don't know if the person in front is going to suddenly swerve to avoid that pothole you didn't see, suddenly brake because they aren't aware you are there, or spit out an insect....
I know that if you want to turn, you stick out whichever arm is required, horizontally. I usually point with my finger too.
Anway, just now on the way home I was being drafted by another cyclist, pretty close, there was heavy rush-hour traffic too, and I needed to stop at the side of the road.
Rather than slamming on the anchors and leaving the bloke behind with nowhere to go, I dug deep into the recesses of my memory to try to remember what my driving instructor told me about hand signals, waaaaaay back when I took my driving test as a wee nipper.
Anyway, my brain decided that the correct memory was that I should stick my left arm out straight, palm facing downwards and flap the arm up and down in a vigorous fashion.
I did so for a few seconds, then began braking while still doing the arm flapping, then got passed within a whisker of being hit by the following cyclist who shouted, "Oi! Knob!" at me, and then something else which I couldn't hear.
Was I right or wrong? Did my brain lie to me?!!!