The Trade and Cooperation Agreement agreed between the UK and the EU in December 2020 allowed personal data to flow freely from the EU (and the EEA) to the UK until adequacy decisions had been adopted, for no more than six months.
In February 2021 the EU published its draft UK adequacy decisions, and we are waiting for these to be formally adopted. There has been some debate at the European Commission, but nothing has been decided yet (at the time of writing this).
The advice from the ICO is that organisations should put in place alternative transfer mechanisms, in case the adequacy decisions are not adopted by the 30th June deadline.
As that June deadline is approaching, you should be finalising these alternative transfer arrangements, which in most cases may mean using Standard Contractual Clauses (“SCC’s”), and you should know which transfers and processors are affected by referring to your Information Audit or RoPA.