1) 40GB plan but downloads between 2am and 9am don't count towards the cap. The same kind of speeds we got overnight on BT would be plenty. Even if it was between 3am and 7am. A small window, but long enough so that you can download 1-2GB overnight.
2) 40GB but instead of the huge charges for extra data, charge $1/GB. So if you want to use an extra 20GB this month, pay an extra $20 (on top of the extra $10 we will have to pay to go from BT to the 40GB plan).
Seems reasonable to me.