The Large Family discount is applied directly on iberia.com. When you've chosen the origin and destination of your flight, if the combination is eligible for the discount the Large Family dropdown menu will appear. Select the relevant category and the flights will appear with the discount already applied. This subsidy is granted by the Spanish government and does not apply if you choose to pay for tickets with Avios.
If you're taking an Air Shuttle flight or a flight with an Iberia code operated by Vueling, you will have to book by telephone on +44 0 20 3003 2109
The Large Family discount only applies if all the passengers in the booking are eligible for it, if the booking only includes domestic routes and if the holder's status is valid until the end date of the trip.
Remember, you will need to show your Large Family Card when you check in or board, so don't forget to bring it with you along with your other travel documents. Iberia will be obliged to deny boarding to passengers with discounted tickets who cannot show their card.