Rule 259
Joining the motorway. When you join the motorway you will normally approach it from a road on the left (a slip road) or from an adjoining motorway. You should give priority to traffic already on the motorway check the traffic on the motorway and match your speed to fit safely into the traffic flow in the left-hand lane not cross solid white lines that separate lanes or use the hard shoulder stay on the slip road if it continues as an extra lane on the motorway remain in the left-hand lane long enough to adjust to the speed of traffic before considering overtaking.