If you wish to become resident in Spain, it is a legal requirement to obtain a TIE (Tarjeta de Identitdad de Extranjero) Card.
A new model was introduced in May 2020 which incorporates a biometric chip in the card, similar to those in modern passports.
This should be distinguished from an NIE number which is required to carry-out many types of official procedures such as purchasing or inheriting a property or opening a business in Spain.
UK citizens wishing to become newly resident in Spain since 1st January 2021 must first obtain a Visa Applications may be made on your behalf by a legally empowered lawyer