Hiya,
In a way you've actually answered your own question. Once you exceed your peak usage allowance, your speeds are restricted until you either pass your billing date, or alternatively you purchase additional usage. This applies to both peak and off peak times.
If you are looking at prepaying for additional usage and looking at your usage patterns over the last few months, perhaps it might be an idea to upgrade to Option 3, which will now give you 30GB at peak?
