The United Kingdom's withdrawal from the European Union can take many forms, including an alternative to a withdraw without an agreement governing relations with the European Union - hard Brexit.
As a result of withdrawal from the European Union (among other things) the status of United Kingdom will change from a Member State to a third country. In the case of hard Brexit, we do not expect the decision of European Commission on the adequacy of the level of protection of personal data would be published in the EU's Official Journal until the day of the UK's withdrawal, mainly because of the time needed to adopt such a decision. The non-existence of the adequacy decision means that the transfer of personal data to the United Kingdom will only be possible if some of the GDPR conditions governing the transfer of personal data to non-EU countries will be fulfilled.
If you are under the effect of GDPR and are transferring the personal data to United Kingdom, you will need to take all appropriate measures to ensure compliance of such transmissions with GDPR in the case of hard Brexit, otherwise you may be exposed to the risk of major sanctions for the non-compliance of personal data processing with GDPR. The transfer of personal data will be possible, for example, through an agreement on the use of approved standard data protection clauses, the existence of approved internal rules containing transmission conditions, adherence to approved codes of conduct or the existence of one of the exceptions for specific situations covered by GDPR.