As I commented previously in another thread:
Thalers bearing the date 1780 are not worth much, too many were made including many made by the London Mint. They were made in London until 1962 when production ceased.
FWIW the Maria Theresa thaler bearing the date of 1780 is a "protected coin" for the purposes of Part II of the UK's Forgery and Counterfeiting Act 1981. Among other things this Act makes counterfeiting legal as long as it's done by the government!