You IP Profile of 17.42 calculates back to 17.998Mbps connection speed. Within the estimates you've posted, but we frequently see near the top end of the "Clean" range.
Something looks to be not quite right but I think it unlikely BT Consumer or BT Wholesale would accept a fault report, and that would stop it even getting to Openreach.
If you sometimes get 20Mbps on speed tests it could be your line is flapping between Fast Path and Interleaved.
Edited by RobertoS (Wed 10-Jun-15 12:11:47)