Those already registered here in Tenerife will at some point over the next year (2020) have to change their police registration document (properly called a Certificado de Registro but also known as a green NIE, green card, residencia, Green certificate) to a third-country nationals’ card, the TIE (tarjeta ID extranjeros). At the time of writing (16/1/20) NO ONE yet knows what the system will be in detail OR when it will come into effect, but information will be published when that becomes known.
* Those who are not yet registered will have until the end of this year to do so (31/12/2020) … BUT … after 31 January there are no guarantees that the qualifying criteria will remain as they are currently, where British citizens are treated as EU member state citizens.
* It is therefore vital to register before 31 January if at all possible … BUT … those who register are those who are coming here to live, and the registration is itself a formal declaration that the person is living in Spain or intends to live in Spain from this point on
* British nationals who just want to retain EU advantages for free movement after Brexit when they just visit for a few months or so at a time sadly simply cannot do so. This is because they cannot register as residents unless they are prepared to make a fraudulent statement to the Spanish immigration authorities that they are living permanently in Spain when they are not
* As things stand, the UK has a transition period which ends on 31 December this year. After this period, British nationals will become subject to the Schengen area restriction of 90 days in a rolling 180 day period. They will not need a visa. Those who are legally resident in Spain (ie registered with the police) will obviously not be considered “visitors” for these purposes.
I will publish updates here but reliable information can be obtained here https://www.lamoncloa.gob.es/lang/en/brexit/howtoprepare/Paginas/190108residence.aspx (scroll down page) and here https://www.gov.uk/guidance/living-in-spain#brexit-what-you-should-do. and here https://www.janetanscombe.com/news/brexit-negotiations-affecting-british-nationals-in-tenerife.html