You are quite right that it is usually the underlying image that is copyrighted If you make a perfect sketch or photograph of a Francis Bacon painting that will be covered by the same copyright as the original painting But for these coins nobody owns the copyright to the actual coin What the British Library claims is copyright over their picture of a coin If the coin was twodimensional we would just ignore them and add a  copyright tag But because it is threedimensional so the lawyer said but isnt an oil painting too then we cant do that But we can sketch their photograph because the sketch is not going to be the same as the original picture It certainly all seems very odd to me but I am not a lawyer  I forgot to say that I got a copy of Rumbles book The Reign of Cnut but I havent had a chance to read it yet so I cant tell you if it is going to be very useful