WiFi is easy to turn off - as BatBoy says let the engineer do the standard install and then disable the WiFi after he has gone. The homehub has a fairly simple interface for configuring WiFi.
If your Sky was previously FTTC as well then the engineer will do very little.
EDIT : Just seen your follow up post. If you have no master then he will fit a master socket with a faceplate splitter. Should take no more than about 10 minutes.
Edited by ian72 (Mon 05-Jan-15 12:42:05)