As the EU deputy spokesperson for the CDU/CSU parliamentary group and rapporteur for affairs in Great Britain, Seif is outspoken on the topic of Brexit. Before the Brexit referendum in June 2016, Seif stated that if Prime Minister Cameron wanted Britain to stay in the EU, he would have to convince his fellow Britons of the advantages of EU membership. Seif compared Cameron to a “butterfly,” and declared that “Britain does not need a butterfly but a lion.”