One Pound GBP in 1979 had the purchasing power of about £3.40 GBP today.
2000
In 1979, the dollar coin issued by the US was the Susan B. Anthony, which has no silver content and no real collector's value, so is a "spender".
The current value depends on condition and variety (there are "flat S" and "clear S" 1979 Kennedy half-dollars--the "clear S" variety is less common and worth more). Without knowing the current value, there is no point in trying to estimate the future value.
It's still worth one dollar.
No coin exists. Silver eagles would not be minted until 1986.
The Royal Mint produced no British Five Pound coins from 1938 to 1979 inclusive.
There were no British 1979 20 Pence coins minted.
2 British Pence in 1979 had the equivalent value of about 1 cent USD.
There were no British Half-Sovereign coins minted from 1938 to 1979 inclusive
There were no British Half-Sovereign coins minted from 1938 to 1979 inclusive.
There were no British Half-Sovereign coins minted from 1938 to 1979 inclusive
There were no British Half-Sovereign coins minted from 1938 to 1979 inclusive
There were no British Half-Sovereigns minted from 1938 to 1979 inclusive.
You need to provide the year. Without knowing the year it is impossible to value your coin. Crowns (5 shilling) pieces were minted in several different years and have vastly different values depending on what year they were minted in.
The Royal Mint did not produce any British Half-Sovereign coins from 1938 to 1979 inclusive.
There was no British 1974 Half-Sovereign minted. The first decimal British Half-Sovereign was minted in 1980.
You are going to have to give a few more details, especially the country of origin because there are multiple currencies called the pound and there are no British 1 pound coins issued that year (they weren't made until 1983), so please post a new question either giving the country of origin or giving enough identifying details that someone could help you identify your coin.