Notably one of the other banks owned by the British government exists specifically to use savings by individuals as a source of capital for the government itself.

https://www.nsandi.com/

[As a result NS&I cannot "fail" in the sense a commercial bank could, your deposits are guaranteed by the continued existence of the United Kingdom, and since it has its own currency this means if the bank fails, the country fails, the currency fails and the money was actually now worthless anyway.]