It would spread cars out from just lanes 2 and 3 thus making better use of the whole motorway. It's used in the USA and doesn't cause Brits too much trouble to get used to. The less changing lanes reduces stress.
Changing lanes on a motorway is one of the most dangerous exercises you can do. At present the rules state you should stay in lane 1 where possible. Thus to overtake a slow car in lane 1 you have to pull out into 2 and perhaps 3 and then back to 2 and back to 1. And you should do this continuously as you reach another car! Everyone doing this according to the rules would actually cause mayhem.
In fact people stay in the lane they are in and just overtake as necessary so part of my proposal is already there. I suggest instead of changing lanes to avoid undertaking so can just continue on until you reach a slower car. For instance if you are in lane 2 and reach a slower car, then if lane 3 is full you move to lane 1 and stay there until you reach a slower car.