One other thing I have been mulling over following on from some of the other posts on here but would divert those discussions too much if I posted on the existing threads...
Plusnet could set a price that averaged out the costs across the different markets and therefore was more expensive for 2/3 but less for 1. This would be much the same as most ISPs are doing that use BT wholesale.
This would have the effect that they would be a little more competitive on M1 exchanges but then when I've looked around they are already pretty competitive and it is difficult to find people to beat on VFM.
They would also be less competitive on M2/3. This means it would be even more difficult for them to go up against the likes of Sky, Be, TalkTalk, etc - all the sorts of providers that would be providing LLU on 2/3 exchanges.
To do this would be very bad for plusnet as it would almost certainly reduce their customer base and therefore their economies of scale.
So, personally I think they do well to offer the different pricing and that although people on Market 1 will feel unfairly treated they probably are already getting a competitive deal compared to a number of other providers (there are providers that are cheaper, some that are more technically savvy, some that have better customer service, etc but as a general provision Plusnet have what I think is a decent balance of all of those - even if we do have to rely on the Plusnet reps on this site to be able to get good support).