Seems quite straightforward to me, BT Wholesale needed a consumer facing CRM system / portal to offer virtual ISP services and chose Plusnet as a partner rather than BT Retail.
You completely miss the point.
BT used to be one business.
Because of their size they were forced to split into three major chunks, Retail, Wholesale and Openreach. Supposedly strong Chinese walls were set up by OfCom to ensure their monopoly position was not abused at the retail and wholesale level as that would have stifled competition.
All relationships between them are supposed to be subject to "equivalence". Hence the ridiculous rigmarole that users like ourselves have to go through with line/broadband faults, tag removal and the like.
The inter-company pricing already seems to be a question of "negotiation" as though there were a normal commercial relationship between them. I wonder how close the prices BT Retail and Plusnet pay to Wholesale are to non-BT ISP prices?
"Ah but, can be said", look at the volume of business - so bigger discounts are reasonable". Sorry, no! In Retails's case that's the whole purpose of the rules, to stop them pricing out the competition, and in Plusnet's case they aren't that big.
Similarly I wonder what Plusnet are charging Wholesale for these "services"?
This looks to me like a "clever" circumnavigation of OfCom anti-monopoly rules. It isn't even being sold by Plusnet, in similar fashion to Post Office phone/broadband or Vodaphone, it is being sold by Wholesale. Odd that! Something to do with Marketing psychology and credibility?
Or looking at your post another way -
this site looks like Wholesale selling Plusnet to the business community. Not the other way round as you suggest. The portal is Wholesale not Plusnet.