Firstly “undertaking” and “passing” are the same things.
To overtake or undertake a lane change isnt always required but your still completing the manoeuvre.
Sitting in the middle lane and calmly continuing past a middle lane hogger is still undertaking.
Rule 268 only gives exemption if the speed of other traffic in other lanes is seriously affected by congestion. Meaning large vehicle numbers. This rules merely covers queuing traffic to allow you to stay in lane and move with it regardless of any others.
If they are only slightly slowed by normal traffic movement then your not exempt.
If the road only has medium to light traffic your expected to move across the lanes as you should.
“Do not” is a definite instruction in applying traffic rules. There is no ambiguity.
Regardless of what speed your travelling in the middle lane (70) your legally required to move over so pointless argument.
It’s easy not to get caught by police as there simply isnt enough. But your forgetting the insurance side of things. Have an accident and it comes out your sneaking up the inside then boom you lose before the case get started.
20 years as a cop. I’m speaking from knowledge and experience.
I’d be more worried about how your insurance company will view it should things go wrong. Especially if your car is packaged with your insurance like with motability. If RSA think your too high a risk a refuse cover where does that leave you.