You may overtake on the left when
You want to go straight ahead when the driver in front of you has moved out and signalled that they intend to turn right.
You have signalled that you intend to turn left.
Traffic in both lanes is moving slowly but traffic in the left-hand lane is moving more quickly than the right-hand lane – for example, in slow-moving stop-start traffic.
You must not overtake when
You are at or near a pelican crossing, zebra crossing or at pedestrian signals.
A traffic sign or road marking prohibits it.
You are approaching a junction.
You are on the approach to a corner, bend, dip in the road, hump-back
bridge, brow of a hill or on a narrow road.
You are in the left-hand lane of a dual carriageway or motorway when
traffic is moving at normal speed.
It would cause danger or inconvenience to another road user.
What to do when somebody overtakes you
Continue at the same pace, unless it becomes unsafe to do so.
Keep as near to the left as is safe.
Do not accelerate.
Be alert in case the overtaking vehicle suddenly pulls back in front of you.