In the event that the United Kingdom leaves the European Union on March 29 without an agreement, regulations have been published recently and they should come into force on Brexit day. According to these regulations, owners of trademarks registered in the European Union will automatically obtain protection in the United Kingdom without any formalities, examinations, or costs. Owners of pending trademark applications in the European Union (as of March 29, 2019) will have a nine-month period to file an application in the United Kingdom if they want to benefit from the priority date of their corresponding trademark application in the European Union.
Source: https://www.inta.org/INTABulletin/Pages/brexit_7403.aspx
