Cut off date for recognising saints
We cannot pick 451 (Council of Chalcedon) as a cut-off date for recognising saints not in visible communion with the Oriental Orthodox family as the whole situation was still flexible in the Middle East for another century and a half. In Britain 451 comes during the period marked by the withdrawal of the Roman legions and the Rescript of the Emperor Honorius (410) telling us to get on with managing our own affairs, which marked a temporary isolation from the wider Roman Empire as we concentrated on repelling the pagan Saxons, so we could hardly be blamed for not getting involved in the aftermath of the Chalcedonian disputes. The fifth and sixth centuries were the rich age of Celtic saints although the Insular Church still retained links with Rome. Whether the Chalcedonian issue had any significance to the church here at that time is very doubtful.
Britain's closest contact during this period was with the Church of Gaul. Although the Gallican Church was part of continental Europe it nevertheless remained quite detached from its Christian neighbours. Professor Ralph Mathisen notes,
"The fifth-century heresies with which the Imperial government and church was so often concerned had scarcely any impact on Gaul. Gauls showed little interest at all in discussing them, and usually did so only when requested to from the outside, and even then only half-heartedly. They preferred to define orthodoxy for themselves, not on the basis of some outside authority." (Ralph W. Mathison, Ecclesiastical Factionalism and Religious Controversy in Fifth-Century Gaul, Catholic University of America Press: 1989, p. xi)
Although the Gallican Church was in full communion with Rome their links were actually quite tenuous. Mathison suggests, "The bishop of Rome learned that, however great his prestige and moral authority might be, he had the ability to influence Gallic affairs only to the extent that he had strong supporters there willing and able to implement his decisions contacts between Rome and Gaul were relatively infrequent ...... the bishops of Rome do not appear to have been overly concerned with Gaul." (op.cit, pp. xiii & xiv)
If this was true of Gaul, how much more of the remote British Isles.