An instrument occurs on an ancient mesopotamian stone carving which has been interpreted as a hammered dulcimer by some scholars. However that may be, the European hammered dulcimer seems to be an independent invention, developed from the psaltery.

The gregorian psalm-tones seem to be stable as far back as we have notation for them. So projecting them back into the period of the British v. English wars is reasonable.

A lyre was recovered from the English burial at Sutton Hoo. This is not too long after sub-Roman period in Britain. Placing a Saxon lyre in the hands of a barbarian character of the Arthurian age is also reasonable. Since lyres were known in the Mediterranean world also, it would be a plausible act of historical reenactment to place a Greek-style lyre in the hands of a British character.

Instruments documented in the Roman world several centuries before the target period are discussed here. Documenting them for sub-Roman Britain may not be possible; but the other side of the same coin is that it is hard to deny the possibility.