Once you've bought something, your contract with the vendor is terminated and (unless the item you've purchased is faulty), they have no further obligation towards you.
Unless goods are faulty, no trader is ever obliged to provide a refund, exchange or credit note. (The only exception would be where you were specifically told that you could return or exchange the item if you weren't happy with it. That statement would then form part of the contract of purchase).
Any trader who gives a refund, exchange or credit note (for non-faulty goods) is doing so purely as a goodwill gesture and not out of any legal obligation.
So, the Wallis staff would have been perfectly entitled to shrug their shoulders and say 'Tough', as they showed you the door.
They've sold the dress and got �35 in their till. They can now only sell the dress for �20, so if they 'gave' you �35 (as the value of the item you can have in exchange), they'd be �15 worse off than before you walked back into the store.
Chris