The new instruction published for 2021 is very simple.
If you, as a British citizen, lived in Spain before December 31, 2020 and you can prove it, you can apply for your residence in Spain from 2021 as if you were an EU citizen (so the process is much easier and faster).
That is, if you already resided in Spanish territory before the end of 2020 (when the legal term was completed), you can welcome you to the withdrawal agreement and get your residence permit.
However, if you cannot prove it or you were not directly in Spanish territory before 2021, in that case from January 1 you are already considered a citizen of a third country (extra-community), and you must apply for your residence permit following the usual procedure.
You shall then find a residence that matches your profile and interests in the country, and perform the entire procedure (longer than the one followed by Europeans).
Here we leave you a list with the different options you have.