I have no trouble believing there was extensive lead mining in England in Roman times, but it's harder to believe it was happening in 750. I can't think of anything the Anglo-Saxons would have been using lead for, and it's hard to imagine there was much of an export trade. I would also be surprised if the Franks were making silver coins on anything like the scale the Romans did.

On the other hand, the lead's there in the data; it must have come from somewhere; so that part of the graph seems to me the most intriguingly mysterious bit.