Legendary helpful member
The latest GOV.UK announcement is sounding promising in parts, but I consider other parts to be very badly worded. Rather than just giving you links, I've picked out the two sections I consider most relevant.
You can sign up for email alerts specifically about living in Spain:
https://www.gov.uk/email-signup/?topic=/world/living-in-spain
The Spanish government has confirmed its intention to protect UK citizens’ rights in the event of the UK leaving the EU without a deal. Issues of residency, social security, healthcare and recognition of academic qualifications will be covered in a “royal decree” (further legislation) expected in February.
They also intend to launch a website on 14 January to provide information for both citizens and businesses.