Drivers in UK are the worst at getting back to the slowest lane (only from my personal experience) so as long as this is acceptable, overtaking from left should be as well, as it is the very reason for this in most cases (not this instance, though).Ignoring the use of the hard shoulder for a moment, he deliberately changed lane in order to overtake on the left ... which will generally get you ticketed (careless driving, or driving without due care & attention) if seen.
The BMW driver is simply inexperienced and maybe a bit 'what do I do on this boring road to make myself look better' or maybe not, nobody knows for sure, from what I see in a video. They drove once or twice on a smart motorway, took it for granted, tried to be clever, got on a hard shoulder, realised mistake ( note that they for some reason accelerated later than they got on a hard shoulder, chances are they did not have an intention to overtake, but had trouble getting back), got embarrassed a bit, then tried to get away with some dignity, overtake few cars and return, but then (too late) they saw, they not gonna make it, slammed on brakes etc. One simple mistake leads to a sequence of chaotic actions. Dangerous, well, very humane as well and has nothing to do with make, as someone stated before. Also, when reading this thread it feels like some live in different UK, I mean, crossing the solid line and overtaking using a hard shoulder... WOW please let me spend a day with you, it must be very busy and exciting.