Nice coin, Biswajit -- It is an old gold sovereign from Great Britain, UK. The 1925 date is not one of the "good" dates, but the coin is still worth its weight in gold.
A gold sovereign contains 0.235 troy ounces of gold. Today's market price of gold is $1879 US dollars per troy ounce. So your coin is worth 0.235 x 1879 = $441 US dollars.
A collector would probably buy it around the gold price of $441. If you sold it to a dealer, he or she would probably offer $300 to $350. A dealer needs to make a profit to stay in business, so that's why the dealer offer is low.