How will the departure of the United Kingdom from the European Union affect its legal situation? 

British citizens living in Spain are welcome to keep their status as EU citizens, therefore having the freedom of movement and labour established within the countries of the Union.

But could the almost 300,000 Britons living in Spain really run out of papers? 
 
It is very unlikely that this will happen. That is to say, they would be left without papers in very few cases and only in the event that the UK's departure from the EU is total and without signing other types of international agreements with the same EU countries or Spain.

Could the British continue to enjoy their same legal status and their same rights in Spain even leaving the European Union? 


 
At present there are countries that, in spite of not being of the European Union, enjoy a statute that allows their nationals to live and work in Spain as if they were community citizens. We are talking about Norway, Liechtenstein and Iceland, and Switzerland. The most predictable in the case of the United Kingdom is that, even if they leave the European Union, they remain within the European Economic Area and therefore their citizens will continue to enjoy free movement. 
 
It seems clear, therefore, that the maintenance of that status is the most likely, either because of accession to the European Economic Area, or because of the signing of agreements with the EU, as is the case with Switzerland. The United Kingdom would be an independent country but with free circulation.