I think there was a short period prior to covid where the online refund tool would recognise a cancellation within 24 hours and process it accordingly.

On a separate note I think to describe this as a strong case or anything like that is very misleading to the OP. I would suggest this is 50:50 at best. The text when clicking through cancellation option on MMB is unambiguous in stating you will receive a voucher. If the OP wants to pursue then I suggest MCOL may be a better route than CEDR.