BRUSSELS—The European Union’s chief Brexit negotiator said Britain must accept the realities of Brexit and it was the UK’s exit from the EU that had caused tensions between London and Brussels over Northern Ireland, not the divorce protocol. Britain has been seeking changes to the Northern Ireland protocol of the Brexit deal after the European Commission sought briefly to prevent vaccines from moving across the open border between EU-member Ireland and Northern Ireland. “Both parties must be conscious of their responsibilities in applying fully this protocol. The difficulties on the island of Ireland are caused by Brexit, not by the protocol,” Michel Barnier told a European Business Summit event on Thursday. “The protocol is the solution.” Just under half of British companies that export goods have run into problems caused by the shift in trade terms with the EU since the start of the year, a survey showed on Thursday. …