It seems possible that Christianity came very early to Britain, shortly after the ascension of Christ, and probably to Glastonbury; at the time of the native Celtic Britons.
Perhaps because of this, and for many centuries, there was historically very little conflict in the British Isles (and no recorded conflict at all in Ireland) between Celtic paganism and the new Christianity.
This had a lasting effect. Because the new Christians were not persecuted by pagans, when Christians became dominant they did not persecute the pagans but assimilated them. Churches were deliberately built on the pagan sacred sites.
And the old pagan deities were downgraded rather than suppressed; to be named among ancient kings, mythic heroes, or even being transformed to saints.
Also Britain was the centre of druidism, the place where continental Celts sent their druids for a very prolonged and difficult education - presumably in colleges of some kind; all this knowledge being memorized and never written (although there was perhaps a written language at the time, using ogham letters)
Druids were leaders of the Celts; with, apparently, a wide-ranging role including ritual, scholarship, divination, healing and warfare. They conducted sacrifices, including human. And these druidic pagans were exceptional in the known world for their strong belief in a personal afterlife - an afterlife in which Men retained their individual identities.
That this belief in an afterlife was strong is evidenced by Julius Caesar, who gave it as the reason for the exceptional courage in battle of the Britons; that it involved retained personal identity may be inferred from the Ancient Briton's practice of guaranteeing loans (IOUs) to be paid-off in this afterlife.
Both of these imply that the Celts knew a lot about the afterlife - in that they had apparently rather exact expectations of what it would be like.
The Celtic expectation of eternal life contrasted with the Greco-Roman and Jewish belief in a afterlife as merely depersonalized ghosts in Hades/ Sheol. The Greco-Romans apparently interpreted the Celtic belief as reincarnation - like the Pythagoreans - but it was more likely a belief in Paradise: Men living eternally in an ideal kind of society and situation (on a magical island to the west, for instance).
This means that when a pagan Celt converted to Christianity and a belief in Heaven it was qualitatively different from the situation for a Roman or Jew. The Roman or Jew expected annihilation of the self - while the Celt expected preservation of the self.
Therefore, for the Roman or Jew Christianity Heaven offered a qualitative transformation of the self, whereas for the Celt it was more like a quantitative enhancement - a divinization - of the self.
I would speculate that this difference in pre-conversion expectations may have led to a difference after conversion. The Greco-Roman or Jew perhaps focused on the new fact of eternal life - and was therefor almost exclusively concerned with salvation. While the Celt would have been more impressed by the enhancement of the self towards becoming a god: a 'Son of God', much like the ascended Jesus - and therefore more focused on theosis - on working to become 'more divine' during mortal life.
At any rate, mainstream Roman/ Western Christianity was rather vague (and indeed uncertain) as to the details of the afterlife; at least until the Mormon revelations from 1830, which began to provide a much more detailed description - and expectations.
But I would guess that the Celtic Christians would have had similarly detailed expectations of Heaven, inherited from their pagan predecessors.
In terms of its organization, Celtic Christianity resembled Eastern Orthodoxy much more than Roman Christianity. Whereas the Roman church was dominated by Bishops and Priests; Orthodoxy is spiritually dominated by Abbots and Monks - and with semi-autonomous hermits inhabiting remote 'desert' regions and performing heroic acts of asceticism (fasts, vigils, prayer), often at the summit of holiness.
But Celtic Christianity was substantially cut off from Constantinople, as well as Rome; and developed along the native lines established very early. These included a great emphasis on mysticism (communion with God) and miracles, with most of the Saints being wonder-workers (most famously Cuthbert).
Although normal in Eastern Orthodoxy, given the pagan assimilation this mysticism may also link up with druidic initiation, which went through many rigorous levels and aiming at spiritual powers that were regarded as magical.
In particular; it may be that the folk heroes of the British may be traced back to those ancient Celtic Gods, assimilating into Christianity. According to the work of Geoffrey Ashe (e.g. Avalonian Quest); Arthur may have derived by many steps from the original primordial titan that was assumed (in the oldest chronicles) to be the ruler of England when it was invaded by Men originally from Troy. This character reappears as the giant Albion in William Blake's prophecies.
Merlin (originally Myrddin) may go back even further, to an even more ancient God who ruled the island of Britain originally. Merlin was originally a place-name for a fortress - but became a personal name. The first recorded name for Britain can be translated as Merlin’s Precinct, or enclosure. The Merlin of literature could have been derived from this ancient god.
By Geoffrey Ashe's account in Merlin: the prophet and his history; the Merlin that we know from literature seems to be derived from a title, role or nature of a person - probably one with prophetic and magical abilities, due a relationship with the ancient god - perhaps as a reincarnation or by divine inspiration.
This led to the the first two known historical Merlins - Ambrosius/ Emrys, the Welsh magician prophet Merlin who mostly contributed to the Arthurian mythos via Geoffrey of Monmouth's History; and Lailoken the later mad prophet Scottish Merlin - who is represented in Geoffrey's later Merlin poem, and who features in the stories of the Scottish Saint Kentigern/ Mungo (the first Bishop of Glasgow, and a missionary of major historical importance).
Another 'Merlin' may have been the Welsh bard Taliesin, author of the oldest surviving native poetry; he is recorded as a companion of Lailoken-Merlin, by Geoffrey.
The Merlin of literature (and his descendants among many other wizards) exerts a continued - perhaps increasing - fascination in Britain (and the Anglosphere more generally). This may have spiritual significance, as some kind of folk-memory of the Celtic Merlin god, via the historical-and-legendary Merlins of the Dark Ages - as assimilated and transformed by the unique nature of Celtic Christianity.
It may even be that - to the Romantic Christian imagination - Merlin is a Celtic name for the archangel of Albion: the angelic being who has been responsible for that ancient and indomitable spiritual reality which underlies modern 'Britain'.