Posted: Mon Jun 8, 2020 10:36am
To apply for residence in Spain you need to be living in Spain, you can't do in advance of that.
If you're not going to move until next year then it's difficult to advise accurately because nobody really knows for sure what it will look like then.
What is for certain is that the financial requirements will become far more demanding, figures like €2400/mth for the first person and maybe €900/mth for a second are being suggested, plus some say significant savings.
Also the the CAB in Spain have this to say:
RESIDENCY / RESIDENCIA
For all residency authorisations, to remain in Spain for periods
larger than 90 days, you will need to apply for a visa from the Spanish
Consulates abroad, and once in Spain, apply for your foreigners ID, the
TIE card.
All residency authorisations will be issued for a limited period of
time, initially, and need to be renewed when they expire. At the
renewal, depending on the kind of residency/work authorisation you
applied for originally, you will need to meet several requisites, some
the same as initially, but special value will be attributed to the
effort you’ve made to integrate. Your knowledge of
various aspects of living and working in Spain will be valued, e.g. with
regards to the Spanish Constitution and Statutes, the values of the EU,
human rights, democracy, tolerance, equality between men and women, and
naturally, knowledge of the official language(s) of your region.
To accredit integration you will need to provide a positive report
from the Autonomous Region you reside in, certifying your assistance at
educational courses relevant to the above mentioned subjects.
Not saying it will necessarily all apply to Brits but as 3rd country nationals it could.