This is a snippet taken from the highway code:
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.
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.
And specifically for motorways:
264: You should always drive in the left-hand lane when the road ahead is clear. If you are overtaking a number of slower-moving vehicles, you should return to the left-hand lane as soon as you are safely past. Slow-moving or speed-restricted vehicles should always remain in the left-hand lane of the carriageway unless overtaking. ...
This does not mean "pull straight into the outside lane until you wish to leave, at which point you cut everyone up getting across to the slip road"
There is NO REASON AT ALL to stay in the middle or outside lane when the lane to your left is clear.
Do you actually realise that you almost halve the traffic a road can carry if you don't pull over? No one can (legally) pass on the left, or even use that half of the road while you poodle along.
Do your part to cut traffic, pull the fuck over.