Here is the section from the Spanish Royal Decree that covers UK Driving Licences, translated:
Within section 4, the purpose of Article 22 is to find an adequate alternative for citizens residing in Spain and holders of a British driving licence, which is currently valid for driving in our country as it is a permit issued by a Member State of the European Union, but that will cease to be so with the withdrawal of the United Kingdom from the European Union.
For this, a transitory period of nine months has been set, during which the holders of a driving licence issued by the British authorities who have acquired their residence in Spain may exchange their driving licence for a Spanish permit, in accordance with the regulations in force in terms of Tráfico, which will allow them to continue driving in our country, despite the withdrawal of the United Kingdom from the European Union.
Notwithstanding the foregoing, during this transitional period of nine months, this exchange will be possible as long as the current system of verification of driving licences established within the European Union is maintained, since if it were not maintained, it would no longer be possible to exchange driving licences issued by the United Kingdom.
After that nine-month period, driving licences issued by the British authorities would be subject to Spanish regulations for driving licences issued by third countries, and the exchange for a Spanish licence will no longer be possible until, in a future, a bilateral convention for the exchange of driving licences with the United Kingdom will be signed.
For the purpose of exchanging driving licences issued in other Member States or in third countries, Articles 18 and 21 of the General Drivers Regulations, approved by Royal Decree 818/2009, of May 8, require holders of the permits have established their official residence in Spain, a concept that is defined in the second additional provision of the aforementioned Regulation.