Closing BT acounts repost from Tech questions

[-=BigD=-]

I've just rang to cancel my BT residential account as the company I work for has installed a BT Business line for me to work from home from.
Now as I was cancelling due to having two BT lines they said they can do it instantly, I'm waiting for a week to I can tell the handful of people who ring me on the landline. But they mentioned that if I was moving to another supplier there would be a 30 day cooling period until it's cancelled. Why???
If I signed up for another, non-BT, residential line over the next few weeks what would happen?
TIA ABers