Rule 138
On a dual carriageway with three or more lanes, you may use the middle lanes or the right-hand lane to overtake but you should return to the middle lanes and then the left-hand lane when it is safe to do so.
SteerTogether · Official English reference
Read rule 138
General driving rules: rules 103–158
Study signals, vehicle control, speed, stopping distances, lane markings and attention while driving.
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