The UK is now in the transition period until December 31, 2020. Since then, it needs to comply with all the EU regulations, including GDPR.
After December 31, 2020, the application of GDPR from the UK organizations will depend.
In fact, GDPR applies to all EU organizations AND to all organizations (wherever they are located) which process personal data of people living in the EU. Therefore, if your organization does not process any personal data of individuals living in the EU (maybe you are a local shop) you may not be under GDPR and you will need to comply with the UK data protection act.
On the contrary, if your activity has a website and it is visited from EU individuals and you sell or buy or work with individuals living in the EU, you will need to comply with GDPR requirements.