American road laws are insane here. The law should be simple; you must be in the outside lane at all times unless you are overtaking, and once you're done overtaking, you should merge back into the outside lane.
As far as I know that’s the law in every state I’ve driven in, but enforcement is pretty much nonexistent. Some states like Texas or Louisiana might have signs reminding people to stay out of the inner lanes except for passing but I’ve never heard of anyone getting a ticket over it. What’s enforcement like in the UK?
That used to be the case in Ireland too, but confusion due to cultural contamination means pretty much everyone moved to numbering lanes (from the "outside"/"slow"/leftmost lane).
When I did my B license test probably about 30 years ago, the Rules of the Road all referenced inside/outside lanes. When I did my CE license last year, it had been updated to only use lanes 1, 2, 3 etc.
Obviously fast and slow are just colloquial terms.
Why? If everyone followed the rules the lanes would segment into slowest on the right, with gradually increasing speed to the left and people moving between the lanes as needed to overtake. It would be far far far better than the chaos of having to move across all the lanes of traffic all the time because there are random campers driving below the speed limit in every single lane.
First, everyone switches right as soon as there's a gap in a righter lane, so lots of unnecessary switching. Second, the right lane is always full making it hard to merge on or off the highway. Third, the leftmost lanes are underutilized when they could be filled with people who have a long way to go until their offramp.
https://www.highwaycodeuk.co.uk/overtaking.html