he may well have a sync of 40meg but his throughput is capped due to something at BTs end. 15 meg is guaranteed throughput from openreach over the gea fibre, what the isp set for throughput is another story (12mb does sound rather familiar). Using the BT tester is the best way to find out what is going on.
They can't. You can't download at your sync speed (and I never claimed you could).
They say: "customers can expect a minimum download speed of 15Mb on its new Infinity packages.". The difference between sync and throughput on the BT speed tester is minimal (a few percent) and would not account for this chap's problem. By "guarantee" I did not mean that they will work to make your line achieve this speed, I just meant that they will not leave you with a BT Infinity connection if they do not get 15Mb minimum sync on their tester