We need to be very careful about attributing various sins to people whose circumstances we are not easily able to judge. It is clear that our separation from the Chalcedonians did not take place in 451 AD, but was a process which extended to the time of the persecution after 518 AD, and much beyond. The last dialogue with the Chalcedonians took place in 565 AD IIRC.
In the West there was very little information about the controversies which were dividing the East, and in the British Isles even less. I do not believe that the British Church had any idea of the detail of the Council of Chalcedon, or of Constantinople 553 AD. Serious studies show that when many in the West rejected Constantinople 553 AD it was because of the flimsiest of knowledge of what had taken place, certainly there was no substantial tranmission of documentary evidence.
If the later British saints were Chalcedonian it was because they understood Chalcedon to have defended the reality of the humanity of Christ - nothing else. But in fact I can remember nothing from the early period of British Church history which even mentions Chalcedon.
It does not seem reasonable or charitable to paint all of the people of the West with the same brush as we might wish to use for certain controversial figures. It also seems to me that the constant tradition of our communion has been that the vast majority of simple faithful people belonging to the Chalcedonian party must not be treated as schismatics or heterodox, but should be received into communion as easily as possible on their rejection of error. Even in the case of priests and bishops there were no great obstacles placed on reception - a rejection of error and a period of probation. Therefore I am not convinced that we should insist on a more rigourous approach, especially in regard to figures who lived 1400 years ago and whose lives we cannot easily judge.
St Eanswythe is an Anglo-Saxon princess who lived a devout life as Abbess of a Monastery near where I live. Her life is entirely a model of that which would be considered saintly, and there is no record of her having ever engaged in Christological controversy which might show her to have accepted any error. Her knowledge of Chalcedon would certainly have been very sketchy indeed. In what sense is she a Chalcedonian - surely only by an accident of birth. Our own bishops have synodally confessed that the Eastern Orthodox have preserved the same Christology as our own, therefore they have synodally confessed that, apart from any evidence to the contrary, St Easnwythe held substantially the same Christology as our own.
Now it is proper to be reserved in commmorating saints in the liturgical Synaxarium until and unless there is a formal reunion, but it is also clear that a great many Coptic priests and bishops, not to mention those from other local Churches, already recognise sanctity in some of those who are outside our formal communion. To be apart from us is not to be a schismatic. The vast majority of folk have no choice about where they are born and often where and how they live. St Eanswythe could never have been Coptic Orthodox - for a great many overwhelming reasons. But it seems to me she lived a holy life in the circumstances God placed her, and showed none of the schismatic spirit which almost delights in a prideful separation.
St Columba lived in the far north and died in 597 AD. I can find no evidence that he ever referred to Chalcedon, or spoke on the Christological controversies. His life, which was written not long after his death, is filled with miracles and the sort of desert spirituality we find among the Desert Fathers, indeed the Coptic monastic spirit was that which inspired the Irish monks. According to most considerations he is certainly a saint, and yet he is also accidentally found living in the Chalcedonian half of the world. Did he ever consider himself a Chalcedonian? I very much doubt it. I don't think it would have crossed his mind to categorise himself by a council which took place on the other side of the world and which he would have considered was entirely a matter of defending the reality of the humanity of Christ.
I am very sure that we know vastly more about Chalcedon than almost anyone living in the West during this time.
It is said that the small Northern town of Berwick was still at war with Russia because of a technicality I will not go into here. Yet in reality this was never so. History is not made up of technicalities. It seems to me that it is the same sort of technicality to say that everyone in the West, and particularly in the British Isles between say 450 and 650 AD was schismatic or heterodox, because of the technicality of birth. During this period and later there were still British pilgrims going to the shrine of St Mina and bringing back the small pottery flasks of holy oil - they have been found in the British Isles. All knowledge of our Orthodox communion would have been as limited as our general knowledge of the Donatists or Novationists. If I asked most of my congregation what they thought of the Quartodecimans they would be as ignorant as most Western people of this period would have been of the actual substance of the Christological controversy and if they thought they knew anything it would be that out communion denied that Christ was human.
This does not seem to me to justify considering anyone in the British Isles a schismatic or heterodox, unless something they can be shown to have said or written points to a genuine, personally held error. Schism is a choice, and I do not believe that the people of the British Isles had a choice, they were simply living the Christian life as best they could on the edge of the world.
The Georgian Church venerates many saints who belonged to our communion. The Eastern Orthodox venerate St Theodora the Empress who belonged to our communion and whose support enabled the Church to be preserved. Certainly St Seraphim of Sarov is so called by many Coptic priests and bishops - and it seems wrong to deny the title since sanctity is not essentially a matter of administration but of divine grace. If, after reunion, we venerate most of the Eastern Orthodox saints, then they are already saints. How much more so those who never participated in the Christological controversies but who manifestly lived a life of grace and holiness.