I volunteer at Oxfam and recently came back from a volunteer conference. Donating your wedding dress is a great idea. 70% of every pound Oxfam raises goes directly into their charity projects. 10% goes to investment in Oxfam's future and 10% goes into funding the organization. Those are really good numbers.
The £250 (about) raised from a wedding dress goes a long way. And funds raised by shops, such as Oxfam Bridal, are really important because Oxfam can use the money without restriction. A lot of times grants and donations can only be used for one specific project, money raised in shops can go directly to crisis areas before the government launches an appeal. It also goes to places which need money but aren't a 'hot topic'.
So yes, if you aren't attached to your dress (planning to make a christening outfit out of it--which is so sweet), and want to use it to make a difference in someone's life, give it to Oxfam!
http://www.oxfam.org.uk/shop/highstreet/bridal.htmOxfam Shop Support on 0845 3000311