You could get over 16Mbps with that downstream attenuation... if it wasn't for your noise margin.
http://www.skyuser.co.uk/forum/route...explained.html
How is the router connected? Via an extension, or direct to the BT master?
Do you have a modern BT NTE5 master socket, with a split front?
Have a read of these:
http://www.skyuser.co.uk/forum/view-master.html
http://www.dslzoneuk.net/socket.php?type=html
... and try the hidden test socket, behind the faceplate.
If that improves your speed / noise margin, then you need to have a look at your wiring, & consider removing the ring wire, or fitting a
BT I-Plate, or fitting a
filtered faceplate.