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 the UK retains the GDPR post-Brexit.
The UK Government has indicated it will implement an equivalent or alternative legal mechanism. It is expected that any such legislation will largely follow the GDPR. This is supported by 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.
