Ummm.. could this not in part have something to do with
OFCOM's Broadband Access Market Classification? The remaining 20CN exchanges are usually Market 1.
Market 1 : [20CN] adsl is provisioned by BT Wholesale only. Their prices are fixed by OFCOM and no cheaper alternative LLU is available.
Its why some ISPs such as Plusnet charge more for Market 1 exchanges, because there are 2 price bandings A&B. Its also why the likes of Sky and TT now aren't interested in taking on any customers who are on Market 1 exchanges. Not only do they deem them not profitable to put in their own kit, but not even profitable enough to take on any customers from them at all.
Its OFCOM that set these rules on pricing. The idea was to encourage LLU providers to go into exchanges and be able to charge less than BT, by forcing BTw to charge a higher rate. Instead what has happened (as anyone with half a clue could guess that LLU would cherry pick) is actually to the detriment to customers on those exchanges because OFCOM are forcing BT to set a higher rate... and the LLU's wont go in because there's no profit in the smaller exchanges.
As I said at the time when OFCOM first brought out the regulation.
Many people will think the above unfair, but those are the rules as defined by the regulatory bodies, which has now resulted in a 'two-tier' broadband in the UK where pricing may likely vary depending upon your location.
I'm sure its OFCOM who decides which exchanges pay Market A and Market B prices.
If AAISP say they are making a loss on those exchanges, then they may have to charge more.
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ETA
Its going to be much harder for the smaller ISP's such as AAISP who will have to maintain a 20CN pipe regardless of the number of users.
If the SP is large enough to have a few thousand users sharing that pipe, then costs to the SP will be cheaper. If AAISP only has a hundred or so still on 20CN, then costs will be exceedingly high.