Monsoon - you can't go wrong.
Are you a guest or the bride?
If you are the bride, it is possible to buy 'off the peg' wedding dresses at any decent bridal shop or at shops like Monsoon who have a fab range these days.
If you're a guest, make sure you know what you look like in the dress from behind. Odds are, you will be in a seat or pew with lots of folks behind you. I can't tell you how many weddings I've gone to where the woman is wearing a dress that looks HORRIBLE on her from the back and I'm sure she wouldn't have bought it if she knew.