There were a pope named Eleutherius (Eleutherus) who held the Holy See from circa 174 to his death in 189, It is said that he received a letter from Lucius, "King of Britain" or "King of the Britons", telling the pope he had the intention to convert to Christianity.
Bede mentioned this story (his source was The Book of the Popes). Then Nennius wrote about it in the 9th century in the History of the Britons, were we can read that a mission, commissioned by the pope, baptised Lucius, "the Britannic king, with all the petty kings of the whole Britannic people". The year of the baptism were set to AD 167, a few years before Elutherius pontificate, and was credited to pope Evaristus (c.99 - c.107).
Geoffrey of Monmouth writes in his History of the Kings of Britain the names of the pope's envoys as Fagan (Phagan) and Duvian (Deruvian).