If you can find someone willing to make that trade, go for it.
5 tuppence is a lot less than a pound, though.
A bank will only ever give you 20 Pence for a 20 Pence coin.
During decimalisation, the value of the pound did not change, only they way it was divided up. A pound coin would be worth the same. However a 1960's pound coin will have been demonetised when the new pound coins where introduced in 1983. This means that officialy coin is worthless. A collector however will likely pay over face value for the coin.
A British 1986 Two Pence coin (Proof FDC), could fetch up to £1 GBP. A reputable coin dealer will be able to give a more accurate valuation. These coins are still potentially in circulation 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 are worth Two Pence.
A British 1982 (7 coin) Uncirculated Mint Set including the 50 Pence down to a Half Pence including the newly issued 20 Pence coin, might fetch up to £9 GBP. There were 205,000 sets issued. A British 1982 (7 coin) Proof Set including the 50 Pence down to a Half Pence including the newly issued 20 Pence coin, might fetch up to £12 GBP. There were 106,800 sets issued. It should be noted that the value of any "coin set" is dependant on it being in its original packaging and condition. 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.
The British 20 Pence coin was first issued in 1982, and general circulation coins have the following specifications - 20 Pence - 21.4 mm diameter, 1.7 mm thick, weigh 5 grams and are made from 84% copper and 16% nickel, and are seven sided or heptagonal. The edge is plain. The British 20 Pence coin has maintained the same specifications since its first issue.
There were no British Five Pound coins minted from 1938 to 1979 inclusive. The coin you possibly refer to is the predecimal British 1965 cupro-nickel Crown (Five Shillings)(Elizabeth II) Churchill Commemorative.
All British 5 Pence coins have a milled edge. The larger pre-1991 5 Pence coins were withdrawn from circulation and demonetised in 1991. 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.
All British 5 Pence coins should have a milled edge. I do not know about "rare", but it is certainly not how it was intended to be, and if there is one, there may be more. It would fall into the category of a minting error and may have some value above the usual. A reputable coin dealer may be able to advise and give a valuation.
7 Men, 49 Women, 44 Children
Nothing, the coin is part of an obsolete currency. Collector values - An Eire (Irish) 1988 cupro-nickel 50 Pence coin (Dublin Millennium), uncirculated and in absolute mint condition could fetch up to £5 GBP. If it has been circulated, not much. An Eire (Irish) 1988 cupro-nickel 50 Pence coin (Dublin Millennium)(Proof FDC), uncirculated and in absolute mint condition could fetch up to £25 GBP. A reputable coin dealer will be able to give a more accurate valuation.
These coins are still potentially in circulation 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 are worth 10 Pence. A British 1996 cupro-nickel 10 Pence coin (Elizabeth II)(Proof FDC), uncirculated and in absolute mint condition could fetch up to £2 GBP. A British 1996 10 Pence coin (Elizabeth II)(Proof in silver FDC), uncirculated and in absolute mint condition could fetch up to £15 GBP. A reputable coin dealer will be able to give a more accurate valuation.
A man walks into a shop, and asks for five pound coins as change for a five pound note. The shopkeeper says, "I'm sorry, we only have one pound coin, I'll give you that and two two pound coins, that should suffice."