After many delays, a lot of confusion and disbelief, Brexit finally happened, thankfully with a withdrawal agreement in place which will allow Spanish citizens already living in UK and British citizens in Spain to keep their rights as they are now.
However this will only happen for those citizens that are already register as EU members living in Spain, which is commonly know as residencia.
In a word, we don't know what is going to happen afterwards, but we do know that any agreements will be reached base on reciprocity.
UK government has recently announced a points system for people wanting to move to UK from Jan 2021, after the implementation period. Anybody wanting to relocate there will have to prove a high English level, a desirable skill or profession, and/or a work contract for at least 31.000€ per year. This will mean the borders will closed for anybody that would like to go there to learn the language and work in a bar, or any other unskilled job.
Based on this information, it is quite possible that the Spanish government will follow the strictest requirements for new British citizens wanting to move to Spain.
At present you need to prove earnings of 550€ a month per person, as a 3rd country citizen you will need 2200€ per month per person, and this just an example. We don't know if a good level of Spanish will be required, but this could be a problem for many people.
If you are already living in Spain and have not yet registered as resident, you should do it as soon as you can, even though you will have to re-register in the future after the implementation period. This way all your right will be protected. If you don't do it you risk not complying with whichever future requirements are asked for in the future and having to go back to UK.
You will always be able to come back to UK whenever you would like to do so, but do not leave this big decision out of your hands.
If you are thinking about relocating to Spain in the future, I would suggest you do so before the end of the year, so you will have the time to register as resident in Spain as per current process.
