Plusnet are indeed owned by BT
One of the things it has enabled BT to do is continue to scam the halfawake regulators at Ofcon.
The majority of the public, like your good self, is unaware that Plusnet is BT Sheffield.
So, if someone looking for a provider has decided that BT are too overpriced, or their overseas customer service is intolerable, or whatever, they can look elsewhere, and quite possibly find that Plusnet appears to offer tolerable service at quite low prices (certainly a lot less than Big BT).
In particular, boutique ISPs from AAISP to Zen look massively expensive in comparison with Plusnet/BT Sheffield, although the service isn't really on the same planet either.
So although Big BT aren't supposed to use predatory pricing against their competition, BT Sheffield apparently still can, as far as the regulators are concerned.