BTw may very well offer that feature to ISPs but not all ISPs take it up (can't be bothered!), EE for 1. Pretty sure EE leaves interleaving entirely in hands of DLM.
Don't think that can be right! Few weekends ago my Fast Path connection went bad, discons, wildly oscillating NM & speed (suspect some BT eng messing about at exchange or cab) but still usable. DLM compensated by applying Interleaving. Didn't bother reporting it to EE as expected it would right itself. Come Mon. connection became solid again. Later that day or next, Fast Path reinstated itself.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 20 Meg WBC