Brigham Young was called to serve a mission while an Apostle in 1839. As specified in Doctrine & Covenants 118, the time and place of departure had already been revealed by the Lord. Mobbers were determined to stop this from coming to pass, but without fear, Brigham Young led the other Apostles to the designated meeting place and they all met as required in preparation to their mission to England. Brigham Young never cowed in the face of his enemies and he was always delivered by the hand of God.
Despite being seriously ill and leaving the care of his family in God’s hands, Brigham Young made his way to New York and then to Liverpool to serve the Lord. While there, he and the other Apostles met with tremendous success. The English people flocked to them and were baptized in droves, with over 800 people baptized in four months. Among that number were lay clergy who converted to the restored Church. Success would continue and miracles would occur just like those in the scriptures.
Brigham Young took the lead in all this, including what he considered his most difficult task: getting the Book of Mormon and hymn book printed in England. Many delays of the printing weighed on his mind, but through his faith and tenacity, they were eventually printed and disseminated to the English people, further increasing conversions and strengthening testimonies. Even harassment and persecution only served to bring more people into the fold.
At this time, Brigham Young as well as the rest of the Apostles instituted the policy of emigration. The newly converted saints in England were encouraged to go to America and join the main body of saints there. This undertaking would serve as excellent preparation for when Brigham would have to lead the whole body of saints out West.
After working hard, Brigham Young was finally able to return to his beloved wife and family. His sacrifice and diligence brought many new saints into the Church and many souls to Christ. He showed the Lord that he was able to feed His sheep and would be ready in the future to guide them to where they needed to be.
For more information see https://www.lds.org/study/ensign/1987/06/brigham-young-in-england?lang=eng