4 important things that the British citizens should do in relation to Brexit:
1. Register as residents requesting an appointment with the Police to obtain the green certificate. Registration at the Town Hall (‘empadronamiento’) is not enough. According to Royal Decree-Law 5/2019 of March 1 a grace period of 21 months will be enabled if the Brexit departure is without agreement or a transition period also of 21 months in case of departure with agreement, so that British citizens can register as residents in Spain. If they do it after the departure date (10/31/2019) they will have to justify that they were already residents before that date by means of receipt of supplies, etc.
2. Enrol correctly at the social security, that is, request the SIP card by applying for the S-1 form at the NHS in UK. You must remember that the European health card is only for holiday makers. Even so, until the end of 2020, the British citizens will continue to receive healthcare, thanks to an agreement between Spain and the UK.
3. Exchange the driving license into a Spanish one. If the exit from the EU is without agreement, there will be a grace period of 9 months to make this exchange. UK will continue to accept all European driving.
4. You must check the validity of your passport. If you only have about 6 months of validity left, you must request renewal. The process must be done online through the following website www.gov.uk
To find out more check these links:
https://www.gov.uk/world/spain, and click in the link about Brexit https://www.gov.uk/world/spain#/world/brexit-spain to register their e-mail to receive relevant information on this topic.
In the following link there is more information about living in Spain https://www.gov.uk/guidance/living-in-spain
You can also find information about Brexit in the British Embassy Facebook page https://www.facebook.com/BritsInSpain/