Great find! I haven't been able to locate another like it yet but my guess is it would be in the patriotic button category and date to the 1811 - 1820 period when the Prince of Wales (George IV) served as Prince Regent.
I've been investigating that angle anyway... it may be something else.
Let us know when you are able to get more of the backmark visible.
For the life of me I can't make out the backmark at all...even with a loupe...it's about as clear as I can get it. It's well worn but there is an S at the 6o'clock position...