It's impossible to quantify Umm because each scenario will differ because of the relative speeds of the vehicles will differ.
E.g. You could justify staying out in lane 2 for a gap between two HGV's that were crawling up a hill which didn't affect your car's speed but which might justify popping back in if you were all on the flat and making good speed.
I think that this problem is why there is no actual offence - it would be impossible to Police.
The real problem lies with those so arrogant that they inhabit the second, third or even fourth lanes whether the inside lane is clear or not.
I was travelling back from Dover recently late at night when a BMW came up the onslip, accelerated hard to join in front of me, cut across the front of me into lane 3 and then slowed down to a modest cruising speed.......
What goes on in their minds?