Just had the following message from Cristina G about voting in the local elections here in May 2019....
UPDATED 20/12/18 - Cristina has now confirmed that anyone who registers on the padrón has the right to vote.
"It seems we will still be able to vote in municipal elections here even if Brexit goes ahead. The elections will be in May 2019. To be eligible to vote you need to be registered on the municipal register (padrón) before 31 December 2018. It is advisable to check whether you are on the padrón before the end of this year."
I understand that this applies for all UK citizens who are resident here. Cristina is checking on the situation for people who own property here but are not resident in Spain. I will provide an update here about this once I have the information.