The GDPR not only applies to organisations located within the EU but it will also apply to organisations located outside of the EU if they offer goods or services to, or monitor the behaviour of, EU data subjects. It applies to all companies processing and holding the personal data of data subjects residing in the European Union, regardless of the company’s location.
If you process data about individuals in the context of selling goods or services to citizens in other EU countries then you will need to comply with the GDPR, irrespective as to whether or not you the UK retains the GDPR post-Brexit. If your activities are limited to the UK, then the position (after the initial exit period) is much less clear. The UK Government has indicated it will implement an equivalent or alternative legal mechanisms. It is a common expectation that any such legislation will largely follow the GDPR, given the support previously provided to the GDPR by the ICO and UK Government as an effective privacy standard, together with the fact that the GDPR provides a clear baseline against which UK business can seek continued access to the EU digital market.