One of the key issues with "right of access" is the issue of unstructured data. Unstructured data can include e-mails, videos, pictures, social media posts and other types of data not organised in a defined manner.
Yes - GDPR will apply to UK businesses post-Brexit. The date for implementation will remain the same as other countries, 25 May 2018.
One of the questions asked about GDPR has been "Does GDPR apply to Jersey and Guernsey?". To clarify this question, let's look at Article 3 - Territorial Scope.