If you want something different every year why not go to a hire shop - that way you can choose a different dress each time and not fork out hundreds of pounds for something that you're only going to wear once...
Don't get anything from Debenhams, it's all so gross and usually lilac. Why don't you try something from monsoon they always have some interesting stuff.
monsoon do have some nice things but I have got 2 lovely dresses from debut which is in Debenhams. One is black and the other is Scarlett and both under 100 quid.