Rules For Bringing A UK Registered Car to Spain?

I think some practical common sense applies here.

Spain is not the sort of country that applies the rules relentlessly. It's a laid back society. Yes the laws are in place but their application on the ground is something very different.

My advice to the OP is do nothing unless you decide to emigrate permanently. Once that decision is made part of the normal process is doing the paper work and engaging in the hassle it creates.

In practice you don't need to watch the calendar. Do it in your own time and be aware of the rules.

Pitby, Billy and his wife are coming here for a holiday in a rented villa. Albeit a long one. But a recce holiday nevertheless to see if it is somewhere they could consider living on a more permanent basis. No one is suggesting breaking , or bending, any rules by the info given so far.  The law states they have 183 days before they must commit and the general  consensus of opinion is ,if they decide to stay, start the process earlier.

Once the initial " how wonderful to have sun everyday" scenario has worn off they may not even stay the course and be eager to return home. If so they will be fortunate to be able to do so because It is obvious by the numbers of Brits desperate to return to Blighty that Spain doesn't suit everyone. In truth I would say only a  very very small percentage of us are actually  happy living here.