So how does that work if you already have a download running that has found the sweet point and then start a second? Or worse try to make a VoIP call.
If another TCP starts then the first will get more loss and back off. TCP isn't fair so what rates they get will depend somewhat on the latency between the endpoints.
Historically at least BT BRAS buffer quite a lot which can be seen as a bad thing - buffer bloat, and is not nice for voip.
As has been said, some ISPs will help here doing Qos for you by setting their rate a bit below BT profile.
IME an ISP that has a policer set at ipprofile rate still fills the bras buffer as they have to set a burst as policers tend to be a bit aggressive otherwise. Policers just drop - no buffer, but in practice they use a virtual buffer = burst.
ISPs using ip profile to do active QOS for you is a good thing - if yours doesn't you can do it your self but being the wrong and of the bottleneck makes it harder and involves sacrificing bandwidth. The slower your line the harder it is.
Of course no ISP using BT wholesale can guarentee QOS like this as at peak times the bras buffers may run below line speed - though this may be rare.
BT wholesale will sell QOS to ISPs to avoid this but it's expensive, I think BT vision uses it - but then I guess expensive doesn't matter so much if you can keep it in the family