He is quite correct to state that pegging one currency to another will eventually fail; the Exchange Rate Mechanism was bound to fail. However, no one has ever speculated on the Scottish or Welsh pound against the English pound, any more than they have speculated on the New York dollar against the Californian dollar.
It must be admitted that there is a cost to monetary union. Banks will lose the huge sums of money which they receive at present for converting funds from one currency to another. This will probably cost jobs. Does anyone know if banks contribute to Tory party funds?