Not sure mate to be honest. I just made the booking online using their own website https://www.brittany-ferries.co.uk/   I guess its because I booked the outbound and return crossings and a cabin for both all at the same time?  I certainly didn't have to click anything special or apply any discount codes etc, I just saw the final total and pulled the trigger. I was also surprised that they only took half the money at the time of the booking, I didn't ask for a split payment or owt special.