No. It is a matter of choice. Nationality is defined by sight of original marriage/birth etc documentation. We do, however, recommend that you register the birth of your child at the British Consulate-General in Madrid if the child is born in Spain or at the Consular section of the British Embassy in Lisbon if they are born in Portugal so that a record will exist in the United Kingdom, especially if you intend to live in the UK in the future.