A dual carriageway is a road which has a central reservation to separate the carriageways.

Rule 137

On a two-lane dual carriageway you should stay in the left-hand lane. Use the right-hand lane for overtaking or turning right. After overtaking, move back to the left-hand lane when it is safe to do so.

Rule 138

On a three-lane dual carriageway, you may use the middle lane or the right-hand lane to overtake but return to the middle and then the left-hand lane when it is safe.