I agree with those above re the correct procedures for roundabouts as per HC , and the wrongness of what I have seen some drivers ( including occupants of liveried driving school cars ) doing .
Re the hand signals : I think a right arm extended outwards , palm open and vertical , is very obviously a right turn signal , and that even the dimmest follower would recognise it for what it is , especially when used to reinforce an earlier given direction indicator/trafficator signal which is still operating .
The ' I intend to slow down or stop ' arm signal is , I think , also fairly intuitive , especially when the vehicle has already started to slow by acceleration sense , and by the time it is given , will be gently on the brakes with brake lights showing ; the advantage of the arm signal is that it ought to convey that the slowing/stopping is a deliberate action , where a turn signal would be inappropriate , and also that it ought to be visible to following traffic beyond the vehicle immediately behind , who may be the only one able to see brake lights ( or any other lights ) on my car .
I don't think lack of understanding on the part of some other drivers should be a reason not to give additional signals ( where the more commonly given ones have already been given , or where something has gone wrong with the signalling devices on a car ) , but it should certainly be a reason to exercise caution and not to rely totally on them .
Use of terminology : when I did my AD training back in the late 70's / early 80's , the police commentary always used the word 'trafficate' or the word 'trafficator' because it was a simple and quite unambiguous term to denote a direction signalling device or its use , even if that device was in fact a flashing light on the corner of the car as opposed to a mechanical semaphore lever which popped out of the B pillar
I still use the term in commentary and in writing even today as it is still clear and unambiguous , compared to 'signal' or 'indicate' - the former of which could mean any number of different actions , the latter slightly less so .
As for waving a whip in the air .... I must admit that one is new to me ( my collection of HC printed copies only goes back to the 1950's ) but if I saw a carriage driver doing it ( and there are annual equestrian events which take place on one of the estates a few miles fro my home ) I think the meaning would be intuitive and I am sure I would know what the meaning was ( thank you anyway for the clarification as one never stops learning ) . Living in the countryside , there are a fair number of horseback riders locally who use the country lanes , and short stretches of the main roads to access them , and who generally give the same hand/arm signals that a cyclist or motorcyclist would . Thankfully the elevated position of horse riders , usually accompanied by hi-viz attire , not only gives them a good view of their surroundings , but makes them easy to see over the top of hedgerows when approaching from around a bend .