Does the GDPR protections still apply to UK citizens after Brexit?

GDPR protections are based on the idea of data subjects, that is people whose data is held by data controllers or processors, who are located in the EU. People who are in the EU have the benefit of the protections of the GDPR. Therefore, if you have UK citizens who are located in the EU after Brexit, they will have the benefit of the GDPR and any businesses who hold data on behalf of any UK citizens located in the EU, after Brexit, will have to comply with the EU GDPR. For example, if you had UK ex-pats who are living in Spain or anywhere in continental Europe or in Ireland after Brexit, they will be located in the EU and as such they will be entitled to the benefit of GDPR. If your business is doing business with such people you will be obliged to comply with the GDPR.