On websites of the British Government, you can already read the agreement concluded between the European Union and the United Kingdom (https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/948119/EU-UK_Trade_and_Cooperation_Agreement_24.12.2020.pdf).
It has to be emphasized that the full title of the document is much longer – as well as the whole document which is not short.
Coming back to the issue of personal data transfer, we should look at the Article FINPROV.10A sec. 1, which stipulates:
For the duration of the specified period, transmission of personal data from the Union to the United Kingdom shall not be considered as transfer to a third country under Union law, provided that the data protection legislation of the United Kingdom on 31 December 2020, as it is saved and This document has been agreed between the European Union and the United Kingdom and is provided for information only. No rights may be derived from it until the date of application. The numbering of the articles is provisional. 407 24.12.2020 incorporated into United Kingdom law by the European Union (Withdrawal) Act 2018 and as modified by the Data Protection, Privacy and Electronic Communications (Amendments etc) (EU Exit) Regulations 201987 (“the applicable data protection regime”), applies and provided that the United Kingdom does not exercise the designated powers without the agreement of the Union within the Partnership Council.
The above means that during the specified period a transfer to the United Kingdom will not be considered as a transfer to third country. Practically it means that the current status quo will be kept.
According to the Article FINPROV.10A sec. 1, the specified period indicated above will end when a decision confirming that the United Kingdom apply adequate security level will be issued. However, the specified period is to last no longer than 4 months (starting from January 1st, 2020) wit possibility to extend by 2 subsequent months.