The transition period ended on 31st December 2020. The UK is no longer implementing EU sanctions. All sanctions regimes will now be implemented through UK regulations.
In the UK you are legally required to report to the Office of Financial Sanctions Implementation (OFSI) if, as a relevant institution, business or profession, you know or suspect that a breach of financial sanctions has occurred.
External accountants are considered relevant professions under financial sanctions legislation. See OFSI guidance for more information on your reporting obligations and how to report to OFSI.
Source: OFSI Financial Sanctions guidance www.gov.uk