Starting in 1983 the UK standardized the 1 pound coin as shown in our pictures. The bust of Queen Elizabeth changes after 1983, and the reverse side changes from time to time. All the coins you find in circulation are made of nickel-brass and are worth face value. You can use xe.com to figure exchange rates between currencies of various countries.
In addition to the business strike coins produced for circulation, the Royal Mint also produced proof coins for collectors. You can see the sharp difference between the proofs and business strikes by considering our secondary picture. Four the the coins are proofs; one is a business strike. It doesn't take a rocket scientist to see the difference!
Now the proofs are made of silver. That changes things. Each one pound proof coin contains 0.283 troy ounces of silver.
Under normal circumstances, the proof coins would be worth their weight in silver, so multiply the current price of silver (found at kitco.com) by 0.283 and you have the value of a silver proof coin. However, circumstances are not completely normal. The Royal Mint intentionally limited the supply of proof coins, making them more difficult to find. This drives the price up somewhat above the silver-only value. Refer to coin catalogs to find approximate values of the proofs. CoinQuest concentrates on coins that are not artificially inflated.
Click to this CoinQuest link for an overview of the value of British coins.