Re: Evidence.
I think we have two definitions of dark age here. AIUI modern historians apply it to a time for which we have few or no sources to tell us what happened. From an English PoV Britannia dropped out of the Western Empire in the early C5th and the external forces at that time were German. Without the Imperial administration only the Church would have been in a position to provide any links with the continent likely to have resulted in surviving documents and the British Church seems to have lost contact before the end of the century. So in these terms the dark age started in the C5th which means that although we know that the Anglo-Saxon settlement followed we have no good account of how it came about. In fact there was no prospect of any records from the Anglo-Saxon side until they were Christianised which mostly fell to the Irish rather than the British (Welsh) Church.
Contact with Rome started to be re-established about 600AD with Augustine's mission & I suppose we can consider it completely established with the Synod of Whitby in 664AD so we have a period of about 200 years with little or no recorded contact with the Mediterranean and for which there was very limited contemporary evidence.
With Bede & Alcuin a recovery was definitely under way. Under Alfred in the C9th there was a well organised administration which, like the Roman, used writing and had a silver coinage so access to gold was of less significance.
As to recovery starting in 1100 an elderly Englishman looking back at that time would have concluded that far from recovering, and Romanesque cathedrals notwithstanding, things had got much worse in his lifetime. And his grandchildren, during the two decades of the Anarchy, would think that they had got worse still.
