Just to wade in a bit here, Openreach do have some forms of traffic shaping available (though I don't know a lot about them) but we don't order any of that when we place orders for the services we use. Essentially that means our prioritisation on the network is the only one.

There is the standard/elevated difference (essentially Residential and Business traffic, though it's not quite that clear cut as you can order both on both services if you wish) but that only comes into effect when a VLAN is particularly busy, in which case pretty much everyone is going to see slowdowns anyway.

Openreach GEA/FTTC/FTTP - SIN 498 Section 2.1.5. Reasonably clear.

WBMC which Plusnet use, (the OP specifically says he is interested in) ADSL2+ - SINs 471 WBMC and possibly 472 WBC. A lot of complex waffle . I'm not surprised you don't know a lot about it. I don't think the Technical Author did either.