The larger pre-1998 50 Pence coins were withdrawn from circulation and demonetised in 1998. So, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they have little or no value.
A British 1978 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II), uncirculated and in absolute mint condition could fetch up to £2 GBP.
A British 1978 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II)(Proof FDC), could fetch up to £3 GBP.
The values quoted are the best possible for the best specimens in their grades, but may be significantly lower due to varying demand for the coin and the prevailing economic climate. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.
The larger pre-1997 50 Pence coins were withdrawn from circulation and demonetised in 1998. So, unless they are part of a Proof or uncirculated mint set or are individual Proof or uncirculated coins and in absolute mint condition, they have little or no value.
A British 1979 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II), uncirculated and in absolute mint condition could fetch up to £2 GBP.
A British 1979 cupro-nickel 50 New Pence coin (heptagonal)(Elizabeth II)(Proof FDC), could fetch up to £3 GBP.
A reputable coin dealer will be able to give a more accurate valuation.
2 British Pence in 1979 had the equivalent value of about 1 cent USD.
There was no 1974 British 25 Pence coin minted.
The British 20 Pence coin was first released in 1982.
Such a coin does not exist. The British 50 Pence coin was first issued in 1969.
There was no 1978 British 25 Pence coin minted.
The first British 20 Pence coin was issued in 1982.
The "face value" of a coin is whatever is inscribed on it, hence the term "face value". The face value of a 10 Pence coin is 10 Pence.
There were no British 1975 25 Pence coins issued.
The first British 50 New Pence coin was issued in 1969.
Your description is applicable to every British coin issued in 1979. On the same side of the coin you read "New Pence", there will be a number indicating "how many" New Pence. The 1979 British 1/2, 5, 10 and 50 New Pence coins have long been withdrawn from circulation and have little or no value unless they are in mint condition, and even then, not much. The 1979 British 1 New Penny and 2 New Pence coins are still potentially in circulation, so are worth 1 and 2 Pence respectively, unless they are in mint condition, and even then, not much.
The value of the copper in a 2 Pence coin would be significantly less then 10% of the face value of the coin at the time the coin was minted. Today, it might be a little more, about 0.2 Pence.
Such a coin does not exist. The British 20 Pence coin was first issued in 1985. They are heptagonal, not round.