Update from the British Embassy in Madrid
As you know, healthcare rights of those living in Spain before 31 December will remain the same for as long as they remain resident here.
Those who have their healthcare funded by the UK (eg S1 holders) will need to apply for a new EHIC card to demonstrate their continued entitlement to EHIC cover from 1 January. The process of doing so is very straightforward. Follow the link here https://www.nhs.uk/using-the-nhs/healthcare-abroad/apply-for-a-free-ehic-european-health-insurance-card/
