Is the deposit owed to the consumer?
That's the crux of it. The key to determining will be who has broken/ is seeking to break the contract agreed when the money was handed over.
The agreement reached and the subsequent timeline isn't 100% clear from the OPs contribution to date.
My reading of it is this (but I am making assumptions to get here, OP please correct me if I'm wrong in any of this):
From the first post, the OP/fiancee put down a 2k deposit to secure a discount off a future purchase. At the point of this transaction, no dress had been chosen, so the contract entered into wasn't for a specific dress, but for a 15% discount off a dress to be chosen at some point in the future.
There was an appointment made to view a selection of dresses, but none were available on the day. The law allows for delays, so this alone is not grounds for a refund unless it can be show that the delays involved were unreasonable.
Again, from the first post, the fiancee subsequently spotted another dress that she prefers, so now wants the deposit back. Change of mind is not a grounds for a refund.
For the deposit to be refundable, the shop would have to be shown as failing to deliver a dress from the chosen designer. Unless the agreement was more specific, any dress from that designer would see them as delivering on their commitment. Are they saying they are unable to fulfill their side of the agreement?