Not enough information. Do you mean a specific, collectible penny that would sell for more than face value to a coin collector, or do you mean the current exchange rate?
If you mean a collectible coin, please post a new question with more details including its date and condition.
If you mean the current exchange rate, it's almost exactly 1p = 2¢
There was no 1974 British 25 Pence coin minted.
There were no British 1979 20 Pence coins minted.
The first British 50 New Pence coin was issued in 1969.
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.
The first British 20 Pence coin was issued in 1982.
There was no 1978 British 25 Pence coin minted.
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.
2 British Pence in 1979 had the equivalent value of about 1 cent USD.
The 10 New Pence coin was not introduced until 1968.
It is in common circulation and worth 2 British Pence, or about 3 cents US/Canadian. It is a common coin and worth no more than face value.