The best that I have managed to come up with is a short article from the ‘Saint wiki’:
May 17 – St. Cathan, Bishop, 6th century.
This saint was probably of Irish nationality. He dwelt for the greater part of his life in the Island of Bute. St. Blaan, whose ruined chapel is still to be seen in Kingarth parish in that island, was his nephew. No particulars of the life of St. Cathan remain to us. His name survives in Kilchatten village, mill and bay, in Kingarth parish, and a hill near is called St. Cathan’s Seat. There is another Kilchattan in Luing Island, Argyllshire, and in the same county is Ardchattan. Churches were dedicated to the saint in the islands of Gigha and Colonsay. The confederation of clans known as Clan Chattan is thought to have originated in Bute, and to have taken its name from St. Cathan. Gillichattan and Macgillichattan are characteristic names belonging to Clan Chattan; the latter was common in Bute in the 17th century. They signify respectively “Servant of Cathan” and “Son of the servant of Cathan.”
As others have mentioned, there is little to nothing known about many of the early Saxon and Celtic saints, and of what is known I suspect a significant amount is embellished or myth/legend, or the victim of ‘Chinese Whispers’.
While Augustine, who was sent from the Pope in Rome to visit King Aethelbert of Kent, is credited with sowing the seed of the religion in Britain, it is also clear that Christianity already had a toehold in England, perhaps as early as 100 AD - with stories and lessons about Jesus being brought over by Roman traders. There is archeological evidence, including what appears to be Roman Christian style burials, and early ‘convert rings’.