Surely as Plusnet are part of BT, then they should have exactly the same coverage as BT?
Yes, but not cuz they are a subsidiary of BT but cuz they use BTw BB exchange kit (as do EE & others).
BT's coverage is nationwide, but not all exchanges are equal; some have ADSL2+, some have ADSL Max only and some have Fibre. Whatever they have BT & PN will sell the same platforms at the same exchange.
Best to work out where you are moving to 1st and check out its exchange.
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