I know nothing about sponge toffee
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I have put the too small leftover pieces of regular toffee in fudge. It was good for a day or two but after that it absorbed moisture and wasn't crunchy anymore. Given the the lack of butter in most sponge toffee, plus the air holes, I would guess it will break down faster.
If your molds make bite sized pieces, I would definitely try it in a few though. Then you can see how long it lasts. If you plan on eating it fast enough it would be a yummy treat.