That would appear to be the halfpenny (it's nearly the size of an old penny, but much thicker, right?). Yours would grade - edge knocks aside - as a clear VF, maybe a bit better. Value around £25, give or take. It was the first official halfpenny for over 20 years, and the first to be minted using Boulton's new machinery. The price of copper saw it reduced in size in 1806, which was a very large issue and much commoner than the 1799. I personally love the design, it's a very handsome coin especially in top grades.