When to avoid overtaking

A dip in the road can conceal a vehicle or other hazard, so it's one place you should not overtake. As you can see in this photo, you can only just see over the crest, and then again the road in the distance. Here is not a good place to overtake.

Other places where overtaking is either illegal or not advised are:

• If it's prohibited, e.g. you'll see this sign
  
• If you can't see at least 100m of clear road ahead of you during the whole overtaking manoeuvre
• If someone else has already started to overtake you
• If overtaking would cause you to break the speed limit
• If you are turning left immediately afterwards and would cause vehicles you've overtaken to have to brake
• If you have to use a median strip, bus lane or bike lane
• If you are crossing a railway crossing
• To the left of another vehicle on a motorway or dual carriageway unless traffic is either moving very slowly in queues, or you are already travelling in the left or middle lane and the lane to the right of you slows down for no apparent reason
• If you have to cross double white lines when the solid line is the one closest to you

For more overtaking rules check this out.