Using the darks to open a account is gambling with your money as it is unclear in which way the whole AEoI/CRS process will impact those accounts. As the account info is faked, you are trusting the reported country does not request more information, and thus get's you account blocked until you perform a form of Identification that you will no be able to perform, thus loosing all your money.
If the EMI does not participate in AEoI/CRS (as most UK based EMI's most like will 'do') there is no reason to use the darks, as you will not be reported anyways.
Back to the︀ UK/GB-IBAN. You need to differentiate between the 'UK internal sort-code' and a SEPA compliant IBAN.︁
Monese, TransferWise and Revolut provide a 'sort-code'. Sogexia provides a SEPA compliant GB-IBAN.