I got my BT Mac code yesterday and gave it to PlusNet.
I then rang BT today to make sure that they knew I was leaving (not just asking for the Mac and not using it).
I used the BT number mentioned in the email they sent with the Mac, 0800 731 6290: very quick reply by somebody with perfect English.
He said no need to explicitly tell BT I was transferring and ending BT contract, that would happen when PlusNet used the Mac to claim the line.
Other people have commented that you should never use the work 'cancel' as that can lead to BT completely cancelling the line: leaving you with reconnection charges and a new telephone number.
BT Infinity 2, moving to PlusNet (26th Sept if all goes well)