Plusnet are part of BT FWIW.
As an ex-BT engineering-type person myself, how 'super' superfast broadband is very much depends on how far you are from the roadside box because that's how much copper is your line and that will slow your connection. Standard BT Infinity is 40Mb/s (megabits not megabytes) at the cab but every metre of copper after that will reduce the speed. Unfortunately, any broadband provider using Openreach's fibre/copper network suffers with the same problem. Virgin are slightly different because they have their own fibre network but this is not available in all areas (Virgin Broadband in non-virgin-fibre areas still uses Openreach's network)