Deep breath, you are totally in the right here.
I doubt you will need it but here is some relevant legalese for you just in case:
Sale of Goods act Section 14
The common one is this:
"(2B)For the purposes of this Act, the quality of goods includes their state and condition and the following (among others) are in appropriate cases aspects of the quality of goods?
(a)fitness for all the purposes for which goods of the kind in question are commonly supplied,
(b)appearance and finish,
(c)freedom from minor defects,
(d)safety, and
(e)durability. "
But the text below is I think pretty relevant to a bra fitting
"Where the seller sells goods in the course of a business and the buyer, expressly or by implication, makes known?
(a)to the seller, or
(b)where the purchase price or part of it is payable by instalments and the goods were previously sold by a credit-broker to the seller, to that credit-broker,
any particular purpose for which the goods are being bought, there is an implied term that the goods supplied under the contract are reasonably fit for that purpose, whether or not that is a purpose for which such goods are commonly supplied, except where the circumstances show that the buyer does not rely, or that it is unreasonable for him to rely, on the skill or judgment of the seller or credit-broker. "
You made it clear that you wanted a bra that fits - they supplied you one that doesn't. So they aren't fit for purpose and given they offer a bra fitting service it was reasonable for you to rely on their skill/judgement.