BT set the 15Mbps limit purely for marketing limits, to keep their "infinity" brand pure. Other ISPs are willing to work to lower limits - and that certainly benefits those who have abysmal speeds at present.
On the Plusnet forum
is an example of someone connected, using FTTC, at over 4km from the cabinet (where the exchange is 8 miles away). The speed is only around 1Mbps (and it took some finessing), but this is a customer who couldn't keep a stable connection at all - and now sounds pretty happy.
BT want to keep infinity as "premium", but it is capable of helping *anyone* if it is allowed to.