On EDF the Direct Debit discount is 6%, which they claim is a typical £62 'saving' a year (i.e. an estimated bill of £1033 a year)
If you overpaid by £1033 in that year you'd lose £36.15 in a decent ISA (3.5%), and it'd work out as costing you even less considering the amount wouldn't necessarily be out of your account for a whole year.
Seems fair enough to me.
Assuming the "estimation" are reasonably accurate, in my experience they are frequently double or even triple actual usage, not to mention I have heard of then increasing the payments with no notice...



