I'm certain it was the backbone that interested BT
Very much unlikely.
(a) it probably wasn't "their" backbone anyway
(b) even if it was, in the grand scheme of things.... goliath BT, david plusnet..... tiny backbone irrelevant to BT
Hint: Technically, they were acquired by BT
Retail. So the internal justification would have been solely sales and revenue based in terms of adding to BT's bottom line.
BT acquired Plusnet in 2007, looking back at one of Plusnet's prior financial reports in 2005, they made the following statement :
PlusNet is BT Retail's preferred partner for projects demanding automated customer relationship management, which is fundamental to Teleworker. Teleworker is a Broadband access solution for organisations to enable their employees to work remotely and allow the organisation to maintain the management of their employees' internet connection. The Teleworker product, which was launched by BT in August 2003, has been adopted by over 30 of BT's blue-chip corporate customers. BT intends to accelerate growth in the number of Teleworker end-users in 2005.
So assuming this is correct, and BT Retail were relying on some of PlusNet's IP and resources to provide services to some of BT's most treasured and critical customers, the acquisition was most likely also a bit of a safety move on the BT to ensure their vested interests in PlusNet's continued existence (and ensuring PlusNet didn't fall into the hands of the competition).