That seems odd. If you are on Sky's FTTC, you have already got a FTTC port anyway. Surely BT would just need to re-route the circuit over the fibre ? Even easier, why not stay with Sky for broadband and phone only - I dont think their charges are any worse than BT's are they? I am also with Sky, and as much as I hate them, I have found their service to be very reliable. We also have another broadband connection to my home ( Virgin ) which has had in the past more slow downs, and drops than my Sky circuit.
Incidentally, I don't use Sky for making any outgoing phone calls, as I use a Sipgate V.O.I.P. line for that, as the call costs are far, far cheaper...............