Such a coin does not exist.
The British 50 Pence coin was first issued in 1969.
There was no 1974 British 25 Pence coin minted.
The British 20 Pence coin was first released in 1982.
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.
Zero. There is no silver in any British general circulation 50 Pence coin, and there has been no silver in any British general circulation coin from 1947 onwards.
There is no silver content in any general circulation British coin from 1947 onwards. The 10 Pence coin is made from 75% copper and 25% nickel.
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.