Such a coin does not exist.
The British 50 New Pence coin was first issued in 1969.
The larger pre-1998 50 Pence coins were withdrawn from circulation and demonetised in 1998.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.
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.
Please check your coin again and post a new, separate question. The term "new pence" was first used in 1968 when Britain began the changeover to decimal coinage.
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.
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.
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.