Have an optional emergency cutout on lines

It's possible for a maxed out FTTC at 40Mbps to download more than 100G of data at peak time in one day. At current excess charges, that equates to about £200. So an out of control download (e.g. a large backup restore of > 1TB) could land you with a excess charge of £2000 in a single month.

I would like to be able to opt in to an emergency cutout switch that throttles my bandwidth to a safe level (e.g. 512K) if my peak hours usage exceeds a certain threshold (e.g. 2 * my monthly units).