Can a homehub 5 work with sky fibre?
Nope.
Do Sky use the same network as BT for fibre?
I currently get nearly 58Mbps with BT and just checked Sky speeds and they said it would be around 40Mbps, seems a bit of a difference.
Broadband gets hammered with console updates/games/YouTube streaming all at the same time.
Sync rates will be identical regardless of which FTTC provider you go with - barring differences relating to quality of modems supplied by the ISP and their willingness to badger Openreach in the case of any dynamic sync rate management problems. Sky have moved to average speeds for some reason, it's no more accurate than the previous figures.