I'm afraid you have got your history regarding this species completely wrong. The Sea Eagle was once widespread in the fenland areas of the UK, as it was in many other areas. It was constant persecution that put the final nail in it's UK coffin in the early 1900's ...
nirofo.
Clearly 'Tideliner' is incorrect to say Sea Eagle is "non-native" in the broader sense although I think he means to refer only to East Anglia. However, to say that 'Tideliner' has got his history wrong and that the species "
was once widespread in the fenland areas of the UK" (implying thereby that it was a widespread
breeding bird) seems to me to be putting the case too strongly.
Yes, there is very strong presumption on
ecological grounds that the species bred in our fenland areas, but
historical evidence for this is thinner on the ground than this comment suggests. In these matters I think you need to make a clear distinction between historical records (sensu stricto) and pre-historic records (otherwise 'native' becomes a meaningless term with rhinos, hippos & the like being 'native'). There is no entirely convincing historical evidence that I'm aware of the the species was a widespread breeder in lowland (= fenland) Britain in historical times (i.e. Roman & post-Roman). There are twenty odd records of archaeological remains of Sea Eagle during the Roman period and a further six during early medieaval times, but only four from late Medieaval period (and about a quarter of all reports refer to birds from upland areas) Despite the widespread records from the Roman period, bones only prove presence, not breeding. (Claims of breeding in the west country and IOW at a much later date are, in my view, entirely spurious and refer to cliff nesting birds noit fenland).
I think, therefore, that the furthest you could go based on the historical/archeaological record is that Sea Eagle was a regular visitor to lowland Britain in Roman times when it
probably bred. If so it may well have persisted in these areas as a breeding species into the early Medieaval period, but seems to have been effectively exterminated in lowland Britain by the later Middle Ages. Victorian records suggest that it continued to be a regular winter vistor well into historical times and occasional attempts to breed cannot be discounted.
Incidentally, a significant number of Medieaval reports appear to come from areas where sheep farming was a significant economic activity (Yorkshire, the Fens, etc) so, given the species reputation, it's hardly surprising that it should have been killed off so early in our history.