Really interesting piece in Fine/Very Fine condition. Elements of the Spanish Colonial host coin are apparent.

United Kingdom 
KingGeorge III (1760-1820)
1804
Silver (.903)
Weight26 g
Diameter41 mm
Thickness1.6 mm
ShapeRound
OrientationMedal alignment ↑↑
DemonetizedYes
ReferencesKM# Tn1, Sp# 3768
Obverse
Laureate and draped bust of King George III facing right, legend around, engraver's initials in raised letters on truncation beneath neck

Lettering: GEORGIUS III DEI GRATIA REX.

Translation: George the Third by the Grace of God King.

Engraver: Conrad Heinrich Küchler 

Reverse
Seated image of Britannia left, holding olive branch in right hand and spear in left, shield bearing the Union flag resting to left, beehive to left, cornucopia to right, all within a garter surmounted by a castellated "crown" of five circular stone turrets. Engraver's initial appears in the triangular space between the sheld, cornucopia, and Britannia's dress.

Lettering: 
BANK OF ENGLAND
FIVE SHILLINGS
DOLLAR
1804

Engraver: Conrad Heinrich Küchler 

Edge
Plain / partially patterned from 8 Real undertype

Comments
Overstruck on Spanish American 8 Real coin issued by the Bank of England with a value of 5 shillings to supplement a deficiency in British regal coinage. The coins were struck by the Soho Mint, Birmingham between 1804 and 1811 though all bear the date 1804. Initially valued at 5 shillings, they were re-valued at 5 shillings 6 pence in 1811 and were withdrawn in 1817-1818. Often found with undertype still showing which adds around 10% of the value dependent upon grade