As far as I am aware the 180 day period starts on the first day of your first visit to an E.U country. So if for example the new law came into effect in 2021, you could make a two day visit to Spain on 1st June, which would trigger the start of the 180 day period. You could then return to Spain later in the year and stay during the months of November and December.
If you then fly back to the UK at the end of December (30th or 31st December) and return to Spain on 1st January, the second 180 day period would then be triggered and you could stay for the months of January, February and March.
My suggestion wouldn't suit everyone but I guess it depends upon the level of inconvenience you are prepared to endure to spend 4/5 months in Spain over winter.
However, as I mentioned in an earlier post it is best to wait and see what develops from the E.U. and UK negotiations in the coming year as things may radically alter
Regards
Richard