For coins dated 1906 to the present, it means the coin was made at the Denver Mint. A "D" mint mark on an old (1838-1861) gold coin means it was made at the long-closed gold coin mint at Dahlonega, GA.
Such a coin does not exist. The first Quarter-Sovereign coin ever produced by the Royal Mint was minted in 2009.
Any collector value would depend on the year and condition of the coin. At the very least, it will be worth whatever the current price of gold might be. A mint condition Sovereign contains 7.322381 grams of gold. Currently gold is at about $1,790 USD per ounce. That makes the gold content of a Sovereign coin worth about $79 USD.
According to coinbrag.com the mint mark is located on the obverse over the date.
mint mark is to the left of the value on the reverse.
$680 to $27,500 depending upon the mint mark and the condition of the coin.
The mint mark on a $5 gold coin from that era is on the reverse (tails) side, just below the eagle.
The mint mark on the US 1849 One Dollar coin is located on the reverse [tails] side of the coin under the wreath. There will be a "C" for Charlotte, North Carolina mint ; a "D" for Dahlonega, Georgia ; an "O" for New Orleans, Louisiana. The absence of a mint mark indicates the coin was struck at the Philadelphia, Pennsylvania mint.
About $1000 for its gold content, regardless of mint mark.
Your coin is either a gold Sovereign or a Half-Sovereign. A Sovereign is about 22mm in diameter and a Half-Sovereign about 19.3mm. The mintmark will be a small M, P or S indicating that the coin was minted at the Melbourne, Perth or Sydney Mint. Coins minted at the London Mint have no mintmark.
If the coin has one, the mintmarks are on the reverse under the eagle.
The mint mark for the 1909 Half Eagle is located on the reverse {eagle} side of the coin just to the left of the arrow points.