they use "download" to differentiate it from "upload", not to imply a difference between sync and throughput which is way too complex for them to handle.
The minimum download of 15M is the minimum predicted downstream rate BT Infinity was provided at, 30M average from the 15 - 40M range of user experiences.
Openreach have a CIR of 20M or line rate on the link from can to exchange, line rate is expressed as ethernet data rate on VDSL, it isn't ATM
MaxDSL - goes as fast as it can and doesn't read the line checker first.
Are your kids pirates ? Limewire, Bearshare, Kazaa, BitTorrent, eMule are all tools of the trade.