If you are in the UK, any bank should be able to change Pounds to Pennies for you.
If you refer to a Bank of England One Pound note, they are no longer in circulation, but the Bank of England has a stated policy of exchanging any withdrawn banknotes for face value.
If you are outside the UK, it is unlikely that you will be able to exchange anything for Pennies, since most Currency Exchange companies do not deal with small change.
The original coinage was in pennies, halfpennies and farthings (quarter pennies). Gradually inflation meant that more and more pennies were needed to buy even smallish items. 240 of those pennies weighed one pound so 'a pound of pennies' was soon abbreviated to 'a pound' and paper notes with a value of one pound (£) were introduced. 240 pennies to one pound continued to be used until 1971 when the British currency was decimalized.
About 181 pennies per pound.
will the NatWest bank change my saved old 20 pound notes if I am one of their customers
Nope - there are only 100 pennies in a pound.
The currency of Britain in 1914 was the (predecimal) British Pound Sterling in which one pound (£1) was equal to 20 shillings (20/-) and each shilling was equal to 12 pennies (12d), so one pound was equal to 240 pennies.
1 pound = 20 shillings, 1 shilling = 12 pennies so 1 pound = 240 pennies.
1 pound equals 100 pennies 10 pounds= 1000 pennies
100 pennies = 1 pound.
240 copper pennies equal a pound.
There are 100 pence in the pound.
240- there were 12 pennies in a shilling and 20 shillings in a pound.
There were 240 pre-decimal pennies in a pound.