About Indicating, brake lights and hazard lights: which statement is WRONG?
Only to avert imminent danger may you give an acoustic or light signal.
The rule of thumb is explicitly three-part, tied to road type: 300 metres (motorway), 200 metres (outside built-up area), 100 metres (inside built-up area).
You always indicate about 30 metres, or roughly 3 seconds, before the turn — that is THE rule of thumb.
That gives other road users no time to react; the rule of thumb is about 100 metres in advance, not at the last moment.
Explanation
trap: flattened rule of thumb. Book p.181 - there is no '30 metres' and no 'seconds' in the book. The real rule of thumb has three branches by road type: about 300 metres on motorways, about 200 metres outside built-up areas, about 100 metres inside built-up areas