1800s? The Roman Empire, and its successors in the Eastern Mediterranean, didn't decline in the 3rd Century.
Wasn't Rome sacked by Goths and Vandals, and then taken over completely by the Goths (Theodoric Amalung, and those guys) in the 5th century (give or take on the dates, my books are in the other room)?
I understand the greater continuity of Byzantium, but did that extend to regular trade as far as Britain?
The 3rd Century was a period of decline. It was rescued by Diocletian and Constantine