can't remember for sure offhand, but I've a feeling they picked up part of the Cable & Wireless / Mercury system after that collapsed
The domestic part of Mercury went - ultimately - to Virgin, but other parts got spun off elsewhere and I have a feeling that Plusnet (under an earlier name) picked up part
Of course I could be totally misrembering, but I'm certain it was the backbone that interested BT. It was at a time when BT were struggling to upgrade their own infrastruture and the Plusnet network made a nice plugin