I don't see why it is such a silly idea. It won't change anything by itself but as
ffsear says, it's just one suggestion of many potential changes which together might help reform the market stalls. Micro billing is becoming more and more common, the transaction fees are getting cheaper and the new contactless technology currently being introduced is fast and specifically designed for payments
under £20. If the markets are going to rely on people buying just £1 of spuds that may well be part of their problem.
Not sure what is so royal about not relying on cash these days - this isn't the 50s.