The larger pre-1993 10 Pence coins were withdrawn from circulation and demonetised in 1993. 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 1983 cupro-nickel 10 Pence coin (Elizabeth II)(Proof FDC), uncirculated and in absolute mint condition could fetch up to £2 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.
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.
The coin is called a "50 Pence" coin. The term "50 New Pence" was last used in 1981.
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.