The UK Bank Charter Act 1844 established that the Bank of England could issue up to £14 million worth of money not backed by bullion. This £14 million was to be backed by "Securities":
And be it enacted, That upon the Thirty-first day of August One thousand eight hundred and forty-four there shall be transferred, appropriated, and set apart by the said Governor and Company to the Issue Department of the Bank of England Securities to the "Value of Fourteen million Pounds, whereof the Debt due by the Public to the said Governor and Company shall be and be deemed a Part;
https://www.legislation.gov.uk/ukpga/Vict/7-8/32/enacted
My questions:
Source, page 73. It mentions other things like how the BoE's note issuance was taxed while local banks were not. So there is a lot more to this. Maybe go to r/AskEconomics or r/EconomicHistory too.
*this is especially true since around the turn of the 19th century at least some debt was issued that would promise an income forever: https://fredblog.stlouisfed.org/2016/07/consols-the-never-ending-bonds/. These would be rather valuable as long as the pound did not depreciate too much.