I have also experienced vehicles joining the motorway and immediately moving into the second lane with no traffic ahead. I think it is also some kind of superiority complex where the 1st lane is considered "the slow lane" or the lane for lorries/trucks, and yes they reached a steady speed of 65mph and held it.. I have also driven on a 5 lane motorway where lanes one and two are litterally empty and you guessed it, three, four and five are chocka-a-block.
I asked a policeman about the legality of overtaking on the left some years ago. There is a difference between undertaking and flow of traffic overtaking on the left. So long as you don't actively change lanes to overtake it is acceptable to pass traffic on your right if their speed is below the flow of traffic.
Rule 268
https://www.gov.uk/guidance/the-highway-code/motorways-253-to-273