Hi HappyChappy. Welcome to the site.
I am not sure if you are aware or not (and if you are, please ignore this post). All UK registered trucks have an electric retarder fitted to the vehicle that does not let them go faster than 56mph, no matter haw much the driver pushes the pedal, once it hits 56mph thats it.
The only way to overtake is to be a heavy truck overtaking a light truck downhill (weight pushes the truck past the limiter) or a light truck passing a heavy one going uphill.
This is where the problem starts. You could be going uphill in a light truck and catching a heavy truck up fast so you overtake, but on doing so, the road starts to flatten out again and the heavy truck gains speed untill both speeds are matched.
With the EU driving regs of 8 hours a day (in basic terms), if you kepts slowing down to let someone in, then, not only would you have to gain your speed again, thus working the engine harder, more often, lowing the fuel ecconomy, and causing more polution, but you might not get to your drop on time, or get home to see your kids etc.
In 99% of cases wagons will let eachother in when back on the flat, as it is easier for them to gain the speed again. All I can reccomend is either you go out for a day with a driver, to see what it can be like, or just do what alot of other motoist do and just overtake in lane 3 and carry on with your commute.
You can also look at another problem from the other side. Car in lane one, clear lane 2 and 3 (with nothing close enough behind to stop car changing lane). Wagon coming up sliproad at 56mph car right next to them wont move over into lane 2 to let them in, resulting in the truck slowing, and causing more people to overtake it as it joins the motorway. I am not saying you do this, but I am just telling you what I see as a car driver on the M-Way every day.
Also, why do some (and I am not saying its you), car drivers pull into the gap between 2 trucks, causing the rear one to slow down, only for the car to pull back out in a few secs to overtake the truck in front.
Or wh do some cars leave it so late to leave the m-way, they nearly take the bumper off of trucks as they cut in front of them?
Anyway, welcome to the site. I hope you enjoy your stay.
Cheers
NB