Giles - Yes there was a lot of forgery throughout the history of coinage but it was especially bad in the late 18th century. There was a major - and very expensive - recoinage in 1799. That was a great thing to find by eyes only. Copper coins don't tend to survive very well. I must have over a hundred from around the early 1600s on and only a very few that are in decent condition. I will post a pic of a lovely George III penny that just happened to survive with a nice dark patina.
LUHE - I am always amazed to think what life was like at the time things I find were lost. I red quite a bit of history to try to get a feel for life at those times. This coin is probably worth about £30. I have never sold one as I enjoy collecting the history.
I have one coin that I think might have some proper value though. It is a Charles I threepence minted at York a few years before he was executed. It is so exceptional in the detail and the condition. It was made by a French moneyer called Briot. It was one of the last "hammered" coins before the advent of milling. I will post a pic below. I was new to detecting when I found it and had no idea how stunning it was. I will get it valued sometime.
BW500 - Thanks for the book suggestion. I love historical novels where the author has done their research. I will look out for that one.

The inscription on the reverse is "CHRISTO AVSPICE REGNO" which means "I reign under the auspices of Christ". That was the attitude that got him beheaded!
The word about the shield is EBOR which is Latin for York.
The Obverse inscription stands for "Charles by the Grace of God King of Great Britain, France and Ireland". This truly is a stunning hammered silver coin. Once in a lifetime find. I found it about a hundred yards from where I am typing this.
Edited to add - Here is one with some damage that sold for $295