The 10 New Pence coin was not introduced until 1968.
There were no British 1979 20 Pence coins minted.
The first British 50 New Pence coin was issued in 1969.
If they have been circulated, all British "New" Pence coins are worth whatever is indicated on them.
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.
2 British Pence in 1979 had the equivalent value of about 1 cent USD.
A British 1970 Shilling (English reverse) (Proof FDC), uncirculated and in absolute mint condition could fetch up to £3 GBP. A British 1970 Shilling (Scottish reverse) (Proof FDC), uncirculated and in absolute mint condition could fetch up to £2 GBP. A reputable coin dealer will be able to give a more accurate valuation.
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.
A shilling was a coin in British currency until 1971. It's value at decimalisation was 5 new pence.
At the decimalisation of Britains currency in 1971, the Halfcrown converted to 12 and a half New Pence.
The term "New Pence" ceased to be used on British coins in 1981. The word "Pence" should have a numeral with it to indicate how many Pence. Pence being the plural of Penny. In 1997 there would have been 50 Pence, 20 Pence, 10 Pence, 5 Pence and 2 Pence coins and a 1 Penny coin.
Pence is the plural of Penny, so you have One Penny or, Two Pence, Five Pence, Fifty Pence, etc. From the introduction of British decimal coins in 1968, the new currency was designated as "New", ie. 1 New Penny, 2 New Pence, etc. until 1981. From 1982 onwards, the "New" was dropped.
Such a coin does not exist. The British - 1 New Penny was first issued in 1971 2 New Pence was first issued in 1971 5 New Pence was first issued in 1968 10 New Pence was first issued in 1968 50 New Pence was first issued in 1969