During the days of the Roman Empire, many Christians practiced in secret despite the threat of death for doing so, and that included Britain. When Constantine converted to Christianity and legalized it throughout the Roman Empire, many people in Britain converted to Christianity.
However, after Hengist and Horsa arrived and conquered, England was ruled by pagans in the various kingdoms established. A century-ish later, roughly in 600, a Kentish monarch (I forget which one off the top of my head) married a Christian princess and allowed her to bring over some missionaries to convert the people, and himself. That eventually led to more missionaries going throughout England to convert the various Anglo-Saxon kingdoms.
However, was there already a Christian presence within England by the time the missionaries arrived? If the Romans had changed the religion of the native Britons to Christianity prior to leaving, and Christians are historically good at hiding their Christianity, would it not stand to reason that there would have already been a Christian presence within England? The particular king that I referenced had married his Christian wife prior to converting as well, so evidently pagans don't seem to hate Christians as much as Christians hate pagans.
A century-ish later, roughly in 600, a Kentish monarch (I forget which one off the top of my head) married a Christian princess and allowed her to bring over some missionaries to convert the people, and himself.
For what it's worth, this isn't quite what happened, and the distinction is an important one. Æthelberht of Kent was married to the Merovingian princess Aldeberge (or Bertha), which can be seen as part of Merovingian attempts to extend overlordship over Kent, and by extension, England. Bertha had her own chaplain, but Æthelberht actually resisted converting at her behest, as this would likely have resulted in Bertha's uncle Chilperic I or cousin Clothar II standing as his godfather, and would have further established symbolic Merovingian overlordship over Kent.
Æthelberht converts as a direct result of the Augustinian mission of 597. Augustine was dispatched by Pope Gregory the Great, who according to Bede, saw English slave boys in Rome and was moved to convert their people in order to spare such proud people an ignoble fate. Likely, Gregory also saw it as his duty to re-establish the Church in England and return it to the fold of Christendom as outlined by the guiding theology of Augustine of Hippo's City of God, and thus to bolster papal authority. Æthelberht converts very quickly, and it's symbolically important that he does this to the papal representative: this allows him to keep his wife happy while also entering Christendom as a triumphant, independent king, rather than as a reluctant satellite of the Merovingians. The Christian Church also allows Æthelberht to rapidly expand the machineries of state at his disposal and increase both his control his territories and the amount of territories he controls.
In this intervening period, the Church in Britain survives largely as the 'Celtic' Church in Wales and Ireland. Christianity in England largely declines as a result of the influx of pagan Anglo-Saxons. Although this is not the mass replacement of people described by Gildas and Procopius, the dissolving of the Roman Imperial state and the establishment of early Anglo-Saxon petty kingdoms makes it much more difficult for the Church to endure. Bear in mind that in the mid-400s, the Churh is still established in England despite the increasing pressure of the Anglo-Saxons, as according to the Vita Germani, St Germanus of Auxerre visits the shrine of St Alban (in St Albans) after combatting the Pelagian heresy in Britain.
The Church declines severely pre-conversion largely as a result of a lack of infrastructure and organisation leading to an inability to maintain or repair buildings, recruit and train clergy or maintain proper practice in the face of a new regime that was at very best indifferent, and may likely have raided churches during a period of conquest, or simply stripped them of land or assets once established. Although Christianity likely survived, it would have been in the form of isolated cults within the Romano-British elements of the population, and likely not at all among the elites.