I'm not specifically a BT broadband fan, although for the past year or so I've posted avidly on the technical help forum on BT Care.
During that time several instances of customers having a fixed rate connection have arisen ... and apart from one time, those connections were shifted successfully over to adslMax ... and some with excellent results. Once those particular connections were reported to the BT moderators on the forum, the relevant escalation process to BTw saw successful results to same. The only one that didn't was a connection in a rural location using a very old aluminium line, and not very condusive to xdsl.
If you can make the connection as good as possible, using a SSFP perhaps, and keeping home impulse noise at bay, DSLAM should try to negotiate the standard 6db noise margin during line training ... and if the line is indeed noise free, should benefit from the maximum line rate that the downstream attenuation permits.
6db margin is the preferred level to achieve maximum rate, which should happen if bitloading is good. Clearly you would have to go to BT for both line and broadband, but I do know that they are proactive in giving you what sounds to be perfectly feasible. It's my opinion that a lot of the fixed line products were just left-overs from days before line adaptive adsl became fully established, and the way BTw set the fault threshold rate for the line and product.
Line IP profiles are strictly for BT line adaptive products on both Max and 21CN, and the IP profile is set at the radius server, according to the line rate you are connected at, and not by any exchange based dslam/msan. Once the line has achieved its connection according to line loop loss and noise margin, which is set for INP, impulse noise protection, the line IP is set at the BRAS, and you get what's left after ATM overhead is catered for.