So what reasons make it acceptable to be cruising on the outside lane of a dual carriageway?
Depends greatly on the road.
Another common one is avoiding unnecessary lane changes, such as when coming up to an entry ramp. Many people like to sit in the outside lane rather than contend with traffic that doesn't know how to get up to speed.
In my post, I was referring to learners where neither of these conditions were met.
But what really annoys me is seeing them in the outside lanes of dual carriageways for no reason, especially when they don't even have a student on board.
Ah.... From the especial remark about there being no student on board, I assumed you meant the instructor (since there is no learner on board, and all).
There was no traffic on the left, and the next possibility to make a right turn wouldn't have been for about 20 miles or so.
Again, that wasn't especially highlighted. Just the implication that there was no good reason you could see.
I now assume you mean an NSL dual carriageway rather than, say, just a two lane road restricted to 40mph going through the centre of town...?
But again, if they want to lane-hog, let them. If you need to get past and can do so without speeding, then it's not undertaking and you're free to do that. If you need to get past and cannot because there's traffic in lane 1, then they are (presumably) overtaking that traffic already.
Don't see what the problem is, unless you're speeding yourself...
How far further ahead? 0.1 miles, 0.5? 1? 5? 10? 50?
Again, depends greatly on the road.
On the example of Reading's IDR, a dual carriageway with a 40mph limit, it's usually easier to stay in whichever lane you will eventually need as you go round it, both to avoid clogging up the numerous entry/exit junctions and because traffic can get pretty heavy pretty suddenly, leaving you trapped in lane, especially approaching the 3-lane roundabouts. It's not as intuitive as it should be and most people claim it takes a few months just to learn how to handle it.
http://www.getreading.co.uk/news/news-opinion/10-experiences-reading-drivers-deal-11052142
In general though, an NSL dual carriageway, I'd think you're fine to start getting into whichever lane you need around 1 mile distant, or once the signs for it start appearing - Whichever is the sooner. Same for the motorway 2-mile warning you get on SatNav and the like, since they're both NSL.