And they would probably get a lot less aggro if they just charged everyone a high price as most of the other ISPs do rather than passing on the lower wholesale costs. Plusnet are currently unique in giving customers the benefit of the BT tiered pricing - but they also get a lot of stick over it.
If Market 1 exchanges constitute 11.7% of the country's exchanges, Plusnet are attracting customers from 88.3% of the remainder. Not to mention a share of the Market 1 customers too.
Would I move if Plusnet charged the Market 1 price for Market 2/3 exchanges. It's very possible. I would have to weight the costs of moving, the price of the opposition, the bandwidth package, the referrals lost (less than £2

) and the usual changing ISP factors.
As already mentioned, Ofcom set the pricing, BT set their wholesale prices accordingly, and Plusnet benfit from their unique position. Plusnet customers also benefit. Are Market 1 exchange customers penalised by being with Plusnet, as opposed to being with another ISP? Not as far as I know.
I'm completely for opening up the Market 1 exchanges for reclassification. That way lies better pricing for all but the wholesalers. Don't forget that I was Market 1 two years ago and was lucky enough to have my exchange reclassified to Market 2, and at the time when Market 2 exchanges were re-evaluated as 'low cost areas'.
Frankly, Ofcom can't really pressure BTw to make the Market 1 exchanges more competitive. How many businesses invite competition without legislation? Equally they can't realistically force the price down, due to the wholsale costs of some of the exchanges.
Back in the mid noughties I was more for pricing based on speed and/or bandwidth. Of course the market expanded quickly without this sort of structure, but only in the higher-density areas. Would 25p per Meg of speed and 25p per GB of download be excessive? Perhaps with a minimum package of £5 for 10GB download.
On my old connection I had 6.5 Meg, which would have been £1.75, and I downloaded say 20GB per month, which would be £5, or a total of £4.25 per month over a basic price of £5, so £9.25 per month.
Someone on 1.3 Meg speeds, downloading the same would pay £8pm. Someone on 75 Meg speeds with 250GB of downloads per month would pay £78.75 per month. Expensive, but said person gets almost 60 times the speed, and uses 12.5 times the capacity. Said person is not paying 60 times the price, or even 12.5 times the price.
With a system of price per Meg and per GB, people would cap at their preferred speed and GB, and web design might be slightly more efficient. See how much webpages have grown in the past ten years:
http://www.websiteoptimization.com/speed/tweak/avera...
Anyway, my preferred system of charging is not in place, and millions wouldn't agree, because they already have max speeds and huge download limits, for as little as Market 1 prices in some cases.