All fibre based services (FTTC, G.Fast, FTTPoD & FTTP) originate from a "head end" exchange, which is likely not to be the serving telephony exchange for the area. The optical fibre component of all those services begins at an Optical Line Terminal (OLT).
An Internet Service Provider (ISP) needs to purchase a (number of) suitably sized "cable link(s)" which is (are) used to connect the ISP equipment to the OLT.
In your case, it appears that BT Wholesale have purchased sufficiently sized cable links to provide all four fibre based services whereas Sky appear to only have purchased sufficiently sized cable links to provide their FTTC services.
I wouldn't be surprised to know if it all comes down to financial expenditure . . .