The claim is here but it is mentioned in a few other places, plus Fr Michael Wood also claims that ‘Aristobulus, Britain’s first bishop (in AD 37) was Orthodox’.
I understand that previous to the Great Schism (1054 CE) there were far more liturgies spread around Europe (including England) and that as the Roman Church has been preeminent for the last 950 years or so in the west, we have forgotten that it might not have been so before that.
It's also known that there was trade with Byzantium in the early middle ages, and even some evidence that Britain was considered part of Byzantium's sphere of influence
So, further to my first question, how strongly orthodox was England in the early middle ages?
I don't think there's a clear historical answer to this, since the split between churches was a process that wasn't complete until long after 1066. Certainly, English and Eastern Christians at the time did not consider themselves part of a single unified church opposed to Roman Christianity.
The theological answer is that the Orthodox church believes it descends from the original, literally orthodox, version of Christianity that once existed throughout Europe. At some point the Roman church fell into error and the two churches split. Before the split, most "Eastern" and "Western" Christians could be considered in some sense Orthodox (excluding Arians, Donatists, and so on).
So Eastern Christians can try to project their currently defined beliefs back on historical churches and say whether they were "Orthodox" or not. They commonly believe that the Roman church fell into error with the addition of the Filioque clause to the Nicene Creed, and that the split became permanent with the "Great Schism" of 1054 (although this certainly wasn't clear at the time).
For example, the English king Edward Martyr is accepted as a saint by the Orthodox, since they believe the Filioque was not in use in England during his lifetime. However, it's debated whether Edward the Confessor should also be considered a saint, partly because he died long after the Filioque was used by the Pope in 1014, and therefore possibly accepted the clause.
In any case, a fringe group of Orthodox theologians argue that the Anglo-Saxon Church was not in communion with Rome in 1054 and was only brought back into line in 1066, when William was given papal blessing for his invasion, apparently because Harold was an oathbreaker and the English Church was corrupt and failing to pay tribute to Rome.
If the Anglo-Saxon church did not accept the Filioque (very unlikely) and was not subject to Rome in 1066, then it could be considered "Orthodox" if you really want to stretch the definition. But it's a pretty silly argument in my view because it requires projecting modern religious definitions back to a time before they really existed. In any case, the Anglo-Saxons did not use the Greek Rite and were certainly more influenced by Rome than Constantinople.