When overtaking is forbidden
Overtaking is prohibited wherever you cannot see the road you would need. That means at bends, on hill crests, at zebra crossings and junctions, in fog, and where a continuous centre line or a no-overtaking sign says so.
It is also prohibited when the vehicle in front is itself overtaking. Two vehicles moving into the same gap is how a head-on collision happens.
The sign for it is circular with a red border - a prohibition, not a warning. Where you see it, the restriction runs until it is lifted.