I don't think a credit note and voucher are the same thing. I had thought that a voucher holder could be considered a creditor initially, but when you think about it, they are not. A credit note gives you a claim on goods that you have had to return or never received, a voucher is effectively a 'good' in itself. From an accounting point of view the holder of a voucher would not appear to me to be a creditor.
Having said that, I feel that Mexx/Mexx International should honour the voucher, have you contacted the ODCA to find out exactly what your rights are in this regard? It does appear that recipients of gifts are not deemed to be 'consumers', so I'm not sure how this affects your case.
This link doesn't give much in the way of info, but worth having a look
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