[8] Savings bank facilities were available for troops at the field post offices set up on the Western Front and elsewhere during the First World War.

In the following years the business of the savings bank expanded, and further premises were acquired to the south, linked to the old warehouse by a bridge across Little Carter Lane.

[13] An extension was built to the north, in Carter Lane, in 1890-94 by Sir Henry Tanner;[14] but soon afterwards work began on a new headquarters in West Kensington: Blythe House, where the Post Office Savings Bank took up residence in 1903.

(The premises on Queen Victoria Street and Carter Lane became a telephone exchange; the Faraday Building now stands on the site).

The Post Office Savings Bank continued to occupy Blythe House until the early 1970s, although it was announced in 1963 that its main centre of operations would be moved to Glasgow.

NS&I sets interest rates both to attract savers and provide low-cost finance for the government, and 100% of any individual's savings are guaranteed by HM Treasury; rules are in place to ensure that it does not offer market-leading products that would stifle competition.

[18] NS&I's head office is within the Department of Education building, in Westminster, London; with operational sites in Blackpool, Glasgow, and Durham.

[19] A 2000 report by the National Audit Office stated that the contract was better value than keeping the operations in-house, and suggested other government departments could learn from the way this public-private partnership was procured and managed.

In November 2023, a six-year customer services contract was awarded to Sopra Steria, another French company, after Atos missed performance targets.