Wholesale Broadband Operator
The simple reason that BT is the country's only (actually almost) broadband wholesaler is simply because Ofcom don't require it of any other network operator. Of course it's completely pointless if there's no local competition anyway. That really means only in areas where VM operate might it be possible, and given the marginal state of their finances, it's not something Ofcom have chosen to push.
You might compare this with the US where network operators are being allowed a long period of grace to implement NGAs without wholesaling requirements. The principle is to encourage investment and not fragment the income to make this viable. The position in the UK is much more hostile if your the incumbent.
In any event, BT taking over EE makes zero difference to the fixed line business and retail market save mobile backhaul where, presumably EE will be favouring OR/BTW fibre over alternatives. As OR (and BTW) have to offer the same products to all customers on the same terms it makes little difference. There is no way at all this returns the market position to that prior to Ofcom being created. Not even close.