Goth
What Hydro said - but here are some numbers and techniques from a PM I recently did for someone else.
Not the full story but if you're doing all I suggest, come back to me and we'll talk some more on motorways.
LANE CHANGE OBSERVATION (Described for right but technique applies to left as well). NOTE that if you can see a vehicle in your mirrors, then it IS behind you (however close). SO ....
driving along, you realise you need to change lanes.
You have (of course!) been keeping an eye on traffic behind using the main (central) mirror, every 5-10 seconds.
To move into the lane on the right, you need to be doing AT LEAST the same speed as the vehicles in that lane. (If you're having to move across because the traffic ahead is too slow, then you need to plan in good time, because otherwise you won't have room/time to accelerate. A good guideline is to aim to start the move when 4 seconds behind the car ahead. Not always possible in busy traffic but the skills will grow. (At 70 mph, 3 seconds is the time between the red/white/blue marker posts along the edge of the hard shoulder)
Check right door mirror for upcoming traffic. Give a signal (not just as a vehicle is about to pass you - you'll give him a heart attack, but just after your indicators are out of his sight), and look fo help from the vehicles behind.
They MAY flash headlights for you, or more probably, you'll see them drop back or at least synchronise their speed with yours. If they are moving across the mirror from left to right, they're gaining on you; if they're moving across the mirror from right to left they're dropping back.[Note to self - must check Ive got that the right way round!]
Most probably you'll see them stay on one place in the right-hand door mirror. They MUST be behind you, so all you have to do is a quick 'shoulder check' to make sure you haven't failed to see someone move into the 'blind spot' alongside your driver's door. If there's no-one there, then gently move across into the lane on the right, keeping your speed but being prepared to slow a bit to increase the gap with the car in front, if necessary, once you're in that lane.
Braking is not a good idea; you may set up a 'traffic wave' of brakelights behind you and you will not then be popular.
JOB DONE.
REMEMBER to do the same for others as appropriate if they need to change lanes. LOOK AHEAD and anticipate their needs. But don't be TOO helpful; if they're not observant enough to see you helping them, they DESERVE to be 'boxed in'. But drive 'defensively', ie keep a weather eye out for the other 'clowns' who move across at the last minute anyway.