If you activate the dead line then you will be in another contract for 12 months for both the phone line and internet with Sky (if you choose them for both line rental and internet).
If you switch the internet to Sky LLU (if available) then I am un-sure on how long the downtime may be (about an hour or less for myself but that was 2 years ago).
If there is a real need to avoid any downtime then you could go with the second line OR perhaps use a 3G dongle for the day.
Downstream ~10 mbps (megabits per second) - ~1250 KBps (kilobytes per second)
Upstream ~2 mbps (megabits per second) - ~250 KBps (kilobytes per second)
Downstream Att. ~62db - SNR ~5.5db - Sync - ~2784 Kbps
Upstream Att. ~31db - SNR ~16db - Sync - ~576 Kbps