There were no British 1979 20 Pence coins minted.
2 British Pence in 1979 had the equivalent value of about 1 cent USD.
Your description is applicable to every British coin issued in 1979. On the same side of the coin you read "New Pence", there will be a number indicating "how many" New Pence. The 1979 British 1/2, 5, 10 and 50 New Pence coins have long been withdrawn from circulation and have little or no value unless they are in mint condition, and even then, not much. The 1979 British 1 New Penny and 2 New Pence coins are still potentially in circulation, so are worth 1 and 2 Pence respectively, unless they are in mint condition, and even then, not much.
The first British 50 New Pence coin was issued in 1969.
The 10 New Pence coin was not introduced until 1968.
If they have been circulated, all British "New" Pence coins are worth whatever is indicated on them.
See the link below.
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.
The larger pre-1993 10 Pence coins were withdrawn from circulation and demonetised in 1993.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 1979 cupro-nickel 10 New Pence coin (Elizabeth II), uncirculated and in absolute mint condition could fetch up to £0.50 GBP. A British 1979 cupro-nickel 10 New Pence coin (Elizabeth II)(Proof FDC), could fetch up to £2 GBP. A reputable coin dealer will be able to give a more accurate valuation.
A British 1979 (silver) Twopence (Proof FDC), uncirculated and in absolute mint condition could fetch up to £20 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.