Re: Just build a wall down the middle of the Channel
Sure, but like then and now there was a different currency in each country. NKR, DKR, SKR. I think there was a similar system between the Rand and the Dollar, and there is now "The Rand is legal tender in the Common Monetary Area between South Africa, Swaziland, Lesotho, and Namibia, although the last three countries do have their own currencies pegged at par with rand."
This has nothing to do with a single currency shared by 19 countries.