There are lots of cheapo cowl-back styles on ASOS. There's a similiar Ghost number for about £200, and it looks fine, but IMHO not a patch on that Biba, which I actually think is stunning and would be a forever dress that would for all smart/formal occasions.
So the question is: can you afford it but don't want to pay that much, or is it not possible to pay that much. Also: have you tried it on yet?
It's a cheap copy, and not nearly as nice as the original. If you can afford it, go for it, it is a classic and you will be able to wear it again or sell it on ebay. You know you will feel a million dollars in it on the night.
Will it be too long though? I see you are not tall..