You most likely have a gold and silver plated 1965 (copper-nickel) British crown (5 shillings) these are incredibly common and the gold/silver plating adds no value to the coin. Your coin is worth 50 cents to a dollar or so.
99 pence
it is around a dollar, over 9 million were minted so they are not that rare
None, such crowns are all copper-nickel and run about $1 in uncirculated condition.
You are probably referring to the 1965 commemorative Crown (a denomination equal to 5 shillings, or a quarter of a British pound) issued in Great Britain. The coin is larger and thicker than a US silver dollar, and there is a portrait of Queen Elizabeth the Second on the front, and a portrait of Winston Churchill on the back. It is worth about a dollar in circulated condition.
The 1965 'Churchill' Crown was mass produced in 1965. 19,640,000 were struck in cupro-nickel (an alloy of copper and nickel). They are worth today (September 2010) between 50p and £1 depending on condition.
It's not a US half dollar; it's a crown (or 1/4 pound or 25 pence) coin from Great Britain. It's worth less than a US in circulated condition, and perhaps US$1 or US$2 in Uncirculated.
£200
Such a coin does not exist. The "Churchill" Crown was a Five Shilling coin issued in 1965. The 1977 Crown was a 25 Pence coin commemorating the Silver Jubilee of Queen Elizabeth II.
Winston Churchill featured on the reverse of the 1965 British Crown, commemorating his death.
It's worth exactly one pound.
Try eBay or a reputable coin dealer. They do not have all that much value , so do not go to too much trouble. British 1965 Crown - Churchill commemorative.
The last British Crown (5 Shillings) coin to be minted was the 1965 Churchill Commemorative.