Two skeletons, probably ethnically Chinese, have been found at a Roman burial site in Southwark in London; they date back to the 2nd to 4th century CE. While the Roman Republic/Empire certainly had contact with China via the Silk Road, and there have been Chinese skeletons found at Roman sites in the Mediterranean, this was the first such find in Britain.
Were they travellers, perhaps an earlier Marco Polo in reverse? What motivated them to travel all the way to such a far flung Roman outpost? How did they die? What else has been found in Southwark, but suppressed by UNIT/Torchwood? Or were they from somewhere even further away than Han era China? A couple of test subjects/fleeing minions of Magnus Greel perhaps? Or time travelling historians/investigators studying the origins of Christianity in Britain/the origins of the Arthurian myth/the revolt of Carausius/the crowning of Constantine? Has anyone analysed the bones for radioactive markers or other signs of a very distant origin?
Lots of potential for scenarios there, everything from a pair of complete red herrings to a major plot. Something that could blend in Lucius Artorius Castus, a possible inspiration for King Arthur (first mentioned in 184CE when he commanded a detachment of Sarmatian conscripts in Britain)