Neither the earlier question (to which I supplied an answer) nor redcrx�s answer really covers the situation which I believe you are describing.
What I understand from your question is the situation where the nearside lane becomes, at some point, solely for vehicles wishing to leave the M99. This is denoted by the markings between lanes one and two becoming thicker than normal, and then leaves lanes two and three (assuming a three lane M99) for vehicles wishing to stay on the motorway.
Many drivers then treat lane one as a separate road and in effect feel free to pass on the inside of those in lane two (as did Bert Biggo in your example). I believe this to be equivalent to overtaking on the nearside. The Highway Code does not specifically cover this situation. However, para 246 is quite clear. It says �do not overtake on the left�. The exception is �...in congested conditions where adjacent lanes of traffic are moving at similar speeds...� I do not believe the situation you describe falls within this exception.
The rationale for my contention is quite simple. Drivers in lane 2 wishing to leave the M99 may still move into lane 1 after the thick dotted lines have started. Although they should have moved to lane one earlier, they may not have done so. Dotted lines may still be crossed and drivers overtaking on the nearside in lane one (even with the presence of the thick dotted lines) would almost certainly be held liable for any collision by their action.
If the motorway is at a standstill (but lane one is not) using lane one to exit the motorway I would say is acceptable (albeit with caution to avoid any drivers moving from lane 2 to lane 1) but it should not be used if going straight on as cutting back into lane 2 when lane one finally becomes separate would be hazardous.