Since leaving the gold standard it basically only means that you can trade in a bill that is out of circulation for some reason for another bill that is.
"The words 'I promise to pay the bearer on demand the sum of five [ten/twenty/fifty] pounds' appears on all of our banknotes. This phrase dates from long ago when our banknotes represented deposits of gold. At that time, a member of the public could exchange one of our banknotes for gold of the same value. For example, a £5 note could be exchanged for five gold coins, called sovereigns.
However, the value of the pound has not been linked to gold for many years, so the meaning of the promise to pay has changed. You can no longer exchange banknotes for gold. Bank of England banknotes can only be exchanged for other Bank of England banknotes of the same face value.
We now maintain public trust in the pound through operating monetary policy."
https://www.bankofengland.co.uk/faq
"The words 'I promise to pay the bearer on demand the sum of five [ten/twenty/fifty] pounds' appears on all of our banknotes. This phrase dates from long ago when our banknotes represented deposits of gold. At that time, a member of the public could exchange one of our banknotes for gold of the same value. For example, a £5 note could be exchanged for five gold coins, called sovereigns.
However, the value of the pound has not been linked to gold for many years, so the meaning of the promise to pay has changed. You can no longer exchange banknotes for gold. Bank of England banknotes can only be exchanged for other Bank of England banknotes of the same face value.
We now maintain public trust in the pound through operating monetary policy."
https://www.bankofengland.co.uk/faq



















