I am not dishing what plusnet and BTw are doing but just I think the IP profiles are not really required to throttle back tcp traffic per connection.
Take a single person running their internet connection downloading over TCP - the amount of data hitting the bottleneck, in this case the MSAN, be it VDSL or ADSL, and the way the data will constantly be filling buffers them emptying then filling buffers again them emptying.
Take the buffers on the MSAN, then add buffers on a LAC along with however many standard routers in the path.
Think about what that excess data will do to the amount of bandwidth they are using, remembering that this bandwidth is measured way upstream from the bottleneck.
At 40-ish quid a Megabit a month across a large user base this rather quickly gets expensive. Far better this gets choked at a BRAS much closer to the ISP.
The IP Profile is not there for the convenience or otherwise of individual users, it is there for the benefit of BT Wholesale, it makes capacity planning easier, avoids cascading buffering from MSAN right through to BRAS, and avoids ISPs paying for data while those buffers fill.
I entirely agree IP profiles aren't required to throttle TCP back per connection, as do BT Wholesale, which is why that isn't their purpose.