I would echo the others, just do it. It is wonderful on the overnight crossing just to wake up and be directly there in Spain.
We didn’t get any vouchers for food even though we had a friends and family discount. Boarding in Portsmouth around midday, I had packed our overnight bags with PJs and toiletries, medications and books, phones and chargers (take an adapter from three pin to European 2 pin). In another bag, I packed the travel kettle that we use in the van, tea bags, mugs and spoons and a flask of cold milk. Add to that snacks like cheese and biscuits and fruit and you are good to go until evening meal. We bought that in restaurant and then next morning, started with cuppas and biscuits until around 11 and then had continental breakfast in the self service cafe.
This kept us going from disembarkation until settled at our first stop with was the free aire at the Elephant Park. Enjoyed watching the elephants that evening and the next morning when they were let out around 9 am and made a run for it!
You have the opportunity now to do it and get a bit of winter sun.