Everything will require clarification. However, I believe that the time spent driving to, in this case, Spain will not count towards the 90 days spent in the EU (Schengen included) However, some form of "immigration" procedure will need to be available to show that you are now entering your visa resident country, and indeed when you leave, be it to return to the UK, or to use some or all of your 90 days in the rest of the EU before returning. Personally, I think it will be very difficult if not impossible to police, and if a self policed system, allowing the traveller to record their own entry and exit dates would be allowed, then that would be wide open to abuse. Really confusing, it is!! 8-)
Except its not confusing. The Schengen system has been in use for 25 years its just we never needed to worry about it as we had free movement as EU citizens. These rules have applied for other third countries since it came into force in the 90s.
The new Schengen database systems came online last year I believe and its very easy to police. Whether you arrive by plane, boat or on foot your registered as entering the zone as soon as you arrive and of course de-registered when you leave. Travelling time to get to Spain will not be added on as a "bit extra".
I dare say if you hung around in Spain all winter nobody would know unless you were unlucky enough to maybe have an accident or get pulled over by plod but the way you would get caught out is when you present yourself at a Schengen border when you come to leave. If your over your 90 days then they will know straight away. People have been getting heavy fines for this for many years and you can be refused entry in the future.
There is a campaign to retain EU citizenship for those that want it (Presumably just those who voted remain I would hope) but it has no chance. This is what the majority of people want though isnt it? To end freedom of movement. Perhaps they thought it wouldnt apply to "Their" Free movement.
You don't seem to have read the posts, this was not about a person traveling around spain in a motorhome, it was about someone with legal authority to be in spain longer than 90 days (i.e. a visa for property) but having no way of 'switching off' Schengen time when crossing from france to spain or visa versa.
They wont have legal authority to stay in Spain though post transition without some kind of residency permit or citizenship I believe or possibly a Padron if they have property. Without some kind of residency the Schengen rules will apply.