what justifies you to be sat in lane 2-3-4 etc if youre not overtaking anyone?
I will give you the benefit of the doubt and assume you mean "you" in general.
No, nothing justifies that. In much the same way as nothing justifies flashing your lights at people or otherwise being an arse.
Matt82 said:the thing that makes my blood boil most is when im trucking along in lane 1, i catch up with another car in lane 2. for me to overtake the socially acceptable way i need to travel across THREE lanes to pass him or just undertake
So undertake if it's safe. It's not illegal.