There was some interesting discussion about DCC here a year ago, including a link to an article about customers get ripped off by it.
In answer to your question, the advice from MoneySavingExpert via that link says:
If the retailer refuses [to void the transaction when you discover you’ve been charged in GBP against your wishes] or you only discover you’ve been wrongly charged later, contact your bank or card issuer. It will then take it up with the retailer’s bank overseas, and may start a chargeback.