Geoffrey of Monmouth (c 1100-1154), that early romancer and historian, claims that in King Arthur's reign, whenever that may have been,
there subsisted at Carleon in Glamorganshire a college of two hundred philosophers, who studied astronomy and other sciences; and who were particularly employed in watching the course of the stars, and predicting events to the king from these observations.
By the time Geoffrey was writing, Christianity had long been established in Britain; but as we have seen, this may well have meant increased knowledge and approval of astrology rather than the reverse.
Can Geoffrey's word be accepted, though? Well, he tells us that his Historia regum Britanniae is a translation of 'a certain very ancient book written in the British language' (that is, Welsh) by Walter, Archdeacon of Oxford. This may have been a simple, individual manuscript; in any event, it has completely vanished. Geoffrey may have invented some of his history, but he would not have invented it all - indeed his often garbled records of some events match with those of which we have knowledge, and he (or his original source) refers often to Cicero, Juvenal, Lucan, Apuleius and others. So the evidence that astrology was in use 'at the time of King Arthur' is worth something, if perhaps not a great deal.
Strands of astrological belief must have been preserved not only by the faint and fading tenets of whatever 'religion' had been supported by the Druids, but in the fading memories of Mithraism, if these communicated themselves to the British, and in the heritage of knowledge left by Rome; and Christianity contributed, too. In The Panegyric of Lludd the Great, a poem written in the 6th century by Taliesin, the 'mythical' British bard, there is a passage, among many dealing with prophesies, which reads
To Britain shall come an exaltation,
Britons of the stock of Rome,
May I be judged by the merciful God.
Astronomers are predicting
Misfortune in the land.
Druids are prophesying
Beyond the sea, beyond Britain,
That the summer shall not be fair ...
