Some practical advice. You might want to sell it on ebay or ricardo.ch. Probably selling it on ebay in the US might make sense...can you send it to someone in the US. It might be complicated to sell it from here and I think the US might be a bigger market. I don't know.
Would you actually want to shorten it and dye it and wear it as something else? Halloween is a few months off... you could be miss havisham. All you need is pip and a rotting cake.... that takes me back to junior high.
Dye it black and be a witch. Or you could be the good witch from wizard of oz (glenda, I think).
While I think there's lots of fun to be had in ripping it to shreds and burning the thing, you might actually make some money and let someone else have the damn thing... assuming you paid for it in the first place.. recoup your investment.
I'd suggest you do a little research first on ebay.com and ebay.co.uk to see if similar dresses are selling and what they sell for. People sell their worn ones all the time.
setting it afire and dropping it into the sea or the lake has some poetic value, too.