Well that's the big question isn't it. The boundaries that differentiate sub Roman Britain from fully fledged Anglo-Saxon England are not well known. I've been involved with some linguistic research on the question, and it would seem that the area around London (called by archaeologists the "core zone") was the first to be subsumed by the new Anglo Saxon material culture. This process alone took about 150 years however, so by the time we get to the 7th century (apparently the dating of this incredible find) you have a British isles divided between the Britons and the English.
It's important when studying ancient history, moreso than modern, to try and dissasociate your mind from your contemporary context. This was an England where the English shared the island with culturally and linguistically distinguishable Britons; only over a period of 500 or so years did the English language finally entrench itself.
So as to whether the gold might be sub-Roman? Probably not, but I wouldn't be surprised if its existance has something to do with the British-English wars...after all there were many British realms, in Wales, a big one at Strathclyde, Dumnonia in Cornwall...
