As the United Kingdom on March 29, 2017 submitted its notification that the UK intends to withdraw from the European Union ("Brexit"), all EU law will cease to apply to the UK from March 30, 2019, 00:00h CET. From the EU perspective the United Kingdom will then become a "third country". "All EU law" of course also includes EU rules on trademarks and Community designs, which means that subject to any transitional agreement that may be contained in a possible withdrawal agreement, the respective EU rules will no longer apply to the UK as of the withdrawal date.
As a result, EU trademarks and Community designs registered in accordance with EU law as well as unregistered Community designs made available to the public in the manner provided for in EU law (Regulation (EC) No. 6/2002) before the withdrawal date will no longer have effect in the United Kingdom:
While at present many uncertainties remain concerning the possible content of a withdrawal agreement, one repercussion of the above, should it come about "full-blast" in its entirety, will be that holders of and applicants for the respective EU rights will lose their trademark and design protection in the UK and will have to act on their own behalf to regain it.
On the other hand, natural or legal persons that are domiciled or have a seat in the UK only will have to be represented before the EUIPO in accordance with Article 120(1) of Regulation (EU) 2017/1001 (on the European trademark) and Article 78(1) of the Regulation (EC) No. 6/2002 (on Community designs) in all proceedings provided for in these two Regulations, other than the filing of an application for an EU trademark or an application for a registered Community design.
Source: Information made available by the EU Commission and the EUIPO on December 1, 2017