Technically it is not up to the merchant but the decision should be yours. Often they take the “liberty” to make the decision on your behalf.
As DCC is something specific to the transaction and not the account you couldnt specify a preference in the application or account-wide, however there were suggestions to prevent DCC by giving the user the ability to block transactions in certain currencies. Unfortunately Revolut does not seem to consider this an important issue.
The principle idea of DCC was probably to give the customer a certain security as to what he is eventually charged, however as the banks chose not to apply the official rate but “invent” some rates the entire concept turned somewhat ad absurdum. With DCC you have a rate guarantee but that guarantee does not only guarantee a fixed rate but also that you pay way more than you’d pay otherwise.