This is appropriate for a British visitor to the museum, it’s likely the head of St. George, the brave knight who fought a dragon that he found somewhere. The stone is actually Roman, but as recycling was quite big in the medieval period, it was repurposed in around 1200 to be used at the Church of St. George in Cologne.
The head fragment was found at the church in 1928 when a major renovation and restoration was taking place and it was likely part of a larger statue. Unfortunately the church was nearly destroyed during the Second World War, but it has now been rebuilt.
The head was once painted and I’ve had AI produce what it thinks it might have looked like. Although I don’t suppose that it did actually look like this, it seems an interesting recreation at least.