Passports are necessary to cross any international border for pets.  This entails having a microchip implanted if not already present, rabies vaccinations and possibly a blood test at least 30 days after the date of vaccination.  We do recommend having 2 rabies vaccinations as a primary course, as there is an approximately 10% failure rate (not achieving 0.5IU) from a single vaccine, and once given, this vaccine is meant to last for 3 years.  Depending on the Brexit deal and regulations, there may then be a 3 month wait before the passport becomes valid for travel.