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 two-dimensional we would just ignore them and add a  copyright tag. But because it is three-dimensional (so the lawyer said, but isn't an oil painting too then?) we can't 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 Rumble's book The Reign of Cnut but I haven't had a chance to read it yet so I can't tell you if it is going to be very useful.   (Talk)