which is why I am asking whether FTTC (fibre to the cabinet) will benefit me if I am with sky when it is implemented by BT
Yes and no.
No, in the sense that having your cab fibre enabled will not
likely enhance your ADSL connection speeds.
Yes, in the sense that, if Sky take advantage of the fibre, you could change your ADSL presentation to FTTC (assuming they let you switch mid contract).
The FTTC delivery dates are fairly vague at best. BT Openreach have a definitive document that has confirmed dates for the next exchanges rolled out, everything else just has a far-future date as an estimate.