Are we talking about VAT or IVA ?
If your in Spain living, you don`t need a IVA number as in the UK, its your NIE number, the same as your CIF.
There is no choice on what number you may or may not use, its not down to individuals to use.
“Charging IVA. Taxable operations
Only businesses and individuals working as self-employed persons can legally charge the VAT on their taxable operations. All businesses, except those that carry out exempt activities, are subject to IVA. All of them must enter the amounts charged in the Spanish Treasury (Hacienda).
IVA taxable operations include the supply of goods, the provision of services and imports”.
“Imposition of VAT on imports and exports operations
Goods and services are subject to VAT tax when these are supplied in Spain. This normally occurs either if the goods are located in Spain at the moment of sale, or if the services’ provider has its place of business in Spain, with certain exceptions.
The imposition of VAT on imports between Member States has ceased. The EU exporter shall quote the recipient’s VAT number on the sales invoice”.