Most coin dealers in Europe buy & sell 19th century gold circulation coins. Mintage numbers have been considerable and premiums often are lower than for many small bullion coins (1/4 or 1/10 oz).
Most gold coins for circulation were 90% gold with 10% copper.
20 Francs (6.45 g) has been popular and was minted in Switzerland, France and Belgium. On the UK forum you will find plenty on Sovereigns. Since the £ was the reserve currency before WW I, Sovereigns may be found across the globe.