OK I think all this needs to be sorted
This is not about ADSL limitations nor is it aobut being 'up to'.
There's a bug with some of BT's equipment not holding its' line configuration. OP needs to raise a fault through his ISP to get his line reset. The BT kit should not even be able to sync this low, and it's failed to take its' configuration properly so needs sorting by BT Wholesale.
It's that simple. It's a fault in the exchange, the ISP needs to raise it with the part of BT that manage the equipment, then it'll get fixed. End of.
If BTBroadband continue to be no help I suggest you go to
http://www.thinkbroadband.com and post on the forum there, you may find that you can get some support from a BTB staffer or one of the staff there who have contacts within BT.
The
only instance where a DSL modem could sync at 128/64k is when it's on a 21CN service, which OP is not. Minimum sync for standard DSL is 256kbps downstream.