Your old gold coin could be a Sovereign or a Half-Sovereign. They are nearly identical other than for size.
The Sovereign is 22.05 mm in diameter.
The Half-Sovereign is 19.3 mm in diameter.
They both have Edward VII on the obverse and St George and the dragon on the reverse.
See the two links below.
Please post a new, separate question with the coin's denomination.
The value of a 20 mark gold coin from 1909 would depend on its condition, rarity, and current market demand. You would need to consult with a numismatic expert or appraiser to get an accurate valuation for this specific coin.
A 1909 Half Eagle can be worth well over $500.00
Theodore Roosevelt 1901-1909 bronze coin valued at?
The Halfcrown is a silver coin. If you have a gold coin it is because somebody plated it. Modified coins have no collector value.
$670.00-$970.00 are the current retail values for this coin.
Assuming the coin is circulated retail prices are $178.00-$300.00
The first gold Fifty Pound coin issued by the Royal Mint was in 1987.
No. The British gold Guinea coin was a coin with a value of 21 Shillings and was 24mm in diameter. The Guinea was last issued in 1813 and was effectively replaced by the Sovereign. The British 22 carat gold Sovereign coin is a coin with a face value of One Pound (or 20 Shillings) and is 22.05mm in diameter. The modern Sovereign was first issued in 1817.
The coin has all writing in the back and his picture in the front with his dates he was in office.....
Such a coin does not exist. In the event that somebody did produce a gold Halfpenny, it would be worth whatever the current bullion value of gold is.
Unless it has an O mint mark, about $300 for its gold content. If it's a 1909-O, it's worth about 2.5X as much as a collectible coin.