I think this is based on a misenterpritation of undertaking.
It would be rediculous for you to stop in an empty lane if, for example, the person in the lane to your right stops. By that measure. If lane 3 becomes congested, and slows to 30mph say, then everyone in lanes 1 and 2 should slow and go no faster than 30 even though the road in front is empty.
This is rediculous.
The definition of undertaking is a deliberate move to a left lane instead if a right lane in order to pass another vehicle.
IMO this is all part of the fact that nobody in the uk has been trained on motorway driving, or passed any form of assessment. Couple that with heavily congested roads and all sorts of strange ideas cone up.
All that happened in your
thinly discized domestic
example was somebody slowed down for no apparent reason.
There are some here that firmly believe that sitting in lane 2 when lane one is empty is perfectly acceptable and their Devine right. Even though keeping left is a major principle of motorway driving, it's 3 points and a fine if caught, and not to mention extremely inconsiderate.
I mention this only because, your person in lane 2 should really be in lane 1.
But then, they've had no training either.