Dangerous Driving is a bit subjective, but undertaking which might be OK in some situations, soon gets silly on a motorway. If you do something which would make a somewhat nervous driver take evasive action, you deserve a ticket.
I reckon there would be a fairly good agreement that what conny describes is not OK.
Precise speeds aren't very important here, surely.
Just because some drivers aren't doing what you might think they should regarding lane choice, is no excuse to be a tit.
My take on this is I won't be forced into the inside lane by bullies if I'm not hogging the outside lane/s.
Real life example. Couple of weeks back I joined a dual carriageway, inside lane. Increased my speed to ~70mph. I was approaching 3 - 4 vehicles on the inside lane that were probably doing ~65mph. On the outside lane, 2-3 vehicles were passing this slower moving group. I pulled out and joined the outer lane group as I wanted to pass the slower moving inside lane group.
Once I pulled out, I realised the outer lane group was taking its time to pass the inner lane group. This was due to the lead vehicle being a bit on the slow side, thus making the overtake manoeuvre a lengthier process. This isn't an unusual occurrence and yes, it can be slightly frustrating.
When I next glanced in my rear view, an old MR2 had settled near my rear bumper, two young guys in it. I'm doing ~70-75mph, this guys sitting a few feet from me. Must have been one of those super-duper drivers that can defy the laws of physics.
He's weaving left, right, getting closer, pulling back, getting closer. He decides to undertake but has to pull out of the manoeuvre due to then being behind one of the slower moving inside lane vehicles. He then settles on my rear bumper again. A gap then appeared on the inside lane so he moved into that, undertook me and the next vehicle, pulled out and settled on the rear bumper of another vehicle. At that point I was exiting the carriageway.
Some reading this will think I should have pulled over at the outset and let the MR2 through. My rationale is I want to pass the slower moving inner lane vehicles and can't be held accountable for the outer lane group moving slower than it ideally should have been.