found this interesting:
"What is the Bank’s “Promise to Pay”?
The words "I promise to pay the bearer on demand the sum of five [ten/twenty/fifty] pounds" date from long ago when our notes represented deposits of gold. At that time, a member of the public could exchange one of our banknotes for gold to the same value. For example, a £5 note could be exchanged for five gold coins, called sovereigns. But the value of the pound has not been linked to gold for many years, so the meaning of the promise to pay has changed. Exchange into gold is no longer possible and Bank of England notes can only be exchanged for other Bank of England notes of the same face value. Public trust in the pound is now maintained by the operation of monetary policy, the objective of which is price stability (http://www.bankofengland.co.uk/banknotes/Pages/about/faqs.aspx).