This coin was issued in three metals: copper-nickel, sterling silver, and 22 karat gold. The first question is, of course, which metal do you have? If in doubt, a jeweler can often help.
Coins made of copper-nickel are worth face value: 5 pounds in the UK. A fully uncirculated specimen may be worth a little more because collectors want to add them to their collections.
Silver and gold coins are worth their weight in precious metal. These coins were issued for collectors to buy, usually at high premiums. If you try to sell one, you will find that the most people will offer is bullion value, sometimes a little less, sometimes a little more, but always close to bullion value. To figure bullion value:
SILVER COINS: contain 0.841 troy ounces silver
GOLD COINS: contain 1.138 troy ounces gold
Go to kitco.com and look up the current price of silver and gold, then multiply by the metal content to get the bullion value. For instance, today's silver price is $17.43 US dollars per troy ounce (tomorrow it will be different, look it up), so a silver 5 pound coin is worth 0.841 x 17.43 = $14.66 US dollars.