"to you all exchanges ARE EXACTLY the SAME COST"
Er, no that's not been correct for the last couple of years or more. Not for Plusnet and not for any other ISP getting their connectivity from BTwholesale.
Since 2008 or so [1] (and based on a review from 2005), so not exactly recent, Ofcom has allowed BTwholesale's pricing to be dependent on the "market type" where the service is delivered. If BTwholesale is the monopoly supplier, they can charge more. Where there is competition, BTw can price more aggressively.
Even before 2008, BTwholesale IPstream pricing had included a somewhat smaller discount in "low cost areas" (a pound and a bit per month off) except for regulatory-obfuscating purposes it had to be called a rebate not a discount.
Readers who want to know more about Plusnet's market-dependent pricing can read more on the Plusnet website e.g.
http://www.plus.net/support/broadband/products/low_c...
Other BTw-based ISPs could also choose to pass this geographic-based pricing on to their customers but off the top of my head I can't think of many (any?) that do. Perhaps there's a question to ask there, although the obvious and often perfectly reasonable answer would be "because it makes our business model too complicated".
[1]
http://www.kitz.co.uk/adsl/broadband_access_market.htm