Sounds like they've followed their T&C as stated on their website:
If you return all or part of an order or purchase, or if we have been unable to fulfil part of your order, that was purchased in whole or in part with a Voucher, your refund will be in the form of a gift card (up to a maximum of the value of your Voucher), with any balance of your refund returned to your original payment method (except where you paid by cash, which will also be refunded in the form of a gift card). Please note that the proportions of your refund that you receive as a gift card and to your original payment method may vary.