I submitted a Ticket. And have been refunded.
Please be advised that when it comes to purchases on Battle.net, all sales are intended to be one-way and final, and therefore refunds or reversals of purchases is not something which Blizzard can guarantee.
With that said though, we do understand that mistakes happen from time to time, as we're all human after all, and so in certain situations we may be able to grant an exception and provide assistance.
In your case, as the expansion has not yet launched, we've agreed to grant you an exception in this situation.
As a result, I have now registered a refund request in the amount of £34.99 GBP and we are proceeding with a few necessary checks and verifications on our own end as it relates to the refund, prior to the funds start to get transferred back to the method of payment you used.
Please note that the estimated time to process a refund varies from 5 - 20 banking days, so don't be alarmed if it takes a few days or even a couple of weeks before the money is returned.