The existing BT socket won't be used, they'll install a new box called an ONT (Optical Network Termination) which can go pretty much anywhere, it doesn't have to be where the current socket is. Depending on the rollout in your area you may get moved over to BT Digital Voice at the same time, that'd mean that your telephone comes down the fibre rather than the copper that's connected to the existing BT socket. My parents have just been moved over to Digital Voice and the voice quality is so much better - the copper into their house is fairly poor.
Wifi is OK, nothing amazing and please don't expect to see anything like 900Mbps through the wifi. If the router is correctly placed then chances are you'll have fairly good coverage.
Gigabit switches are pennies, so time to go shopping