I was thinking about selling mine, it was £1200 new and I'd hate to loose it on ebay for something silly like £50. I have thought about having it turned into my little boy's Baptism outfit as it's got a long train on it so plenty of material but I would need to look out a decent seamstress and suss out costs, I don't have a pattern either.
I agree you could sell in your local paper, the local friday-ad, put it on ebay with a reserve price, if you started bidding even at £20 I'm sure you'd find a winner (a lot of people are apprehensive about buying fom overseas, Hong Kong etc) you could place the reserve at £50 which I'm sure it would reach then you have your money back, if it makes more it's a bonus - if it doesnt reach the reserve price you could then decide whether to offer it to the highest bidder, re-list it or sell elsewhere.
I did see something on the news last week saying Oxfam are buying/selling wedding dresses now with all proceeds going to good causes they help. They sell them for around the £250 marker and have a 6 month back list of people booked in for appointments/viewings etc. They only have for sale what's on the rack so you couldn't decide on the dress you loved but in a different size or colour type thing but perhaps that's worth looking into?
Jax I think it's lovely that you donated your happy dress to charity, that's really nice I'm sure they were thrilled with it.