Above link to an article on an ACA parish homepage included for interest.
What is interesting about this is the date of the Litany - in its present form, no later than AD 889. The Litany prays for the King Gregory of Scotland as he still rules (being Giric, known as 'King Gregory the Great', murdered in AD 889.) What makes this interesting is that the beginning point for those who denounce the claims of St. Joseph of Arimathea (and St. Aristobulus) mission to Brittania stand upon the late 19th c. claim that William of Malmesbury 'invented' the story around 1125... 235 years after the Litany of Dunkeld took its final form.
The early Church in Britain centered around the Silurian capital at Caerleon (near present day Cardiff) which was at the time a new Roman fort. The site of Glastonbury was important as being where the Gospel is supposed have first been preached in the 'Extreme Occident' and the foundation of what would become the basis of Celtic/Saxon monasticism. (Note - St. Joseph of Arimathea by tradition was only presbyter, not episcopus.)
The list of those who attest to the Apostolic origin of the Church in Britain includes Eusebius of Caesarea 3rd - 4th c., St. Hilary of Poitiers 4th c., St. Gildas the Wise 5th - 6th c. , St. Augustine of Canterbury 7th c., St. Nicephorus of Constantinople 8th - 9th c., Blessed Maurus Rabanus of Mainz 8th - 9th c. (and other documents of lesser mention.)
Those who attest to St. Joseph of Arimathea as the first father in Britain, include the later western 'Father of Ecclesiastical History' the Venerable Cardinal Cesare Baronius of Naples 16th - 17th c., and the Jesuit Melchior Inchofer 16th - 17th c., and the earlier St. Gregory of Tours 6th c, St. Isidore of Seville 7th c., as well as various Welsh and English medieval documents*, or the claims of John of Glastonbury 14th c. and John Capgrave 14th c. as to seeing the same claims from the Holy Emperor Theodosius 4th c.
* As with the Fathers, many documents are quoted by various authors, but no longer are extant - though a copy might exist somewhere. According to various authorities, there are still thousands of untranslated documents in various medieval and ancient languages in the monasteries, libraries, and private collections of Europe.