National Savings and Investments (ns&i) is backed by HM Treasury. They are the governmet's savings bank (they were originally the Post Office Savings Bank). When you deposit with them, they lend your money to the Government, so their rates reflect whatever the Government is willing to pay from time to time to borrow money. If the Government can borrow money cheaper (say, from another government), then ns&i will drop their rate so that it remains competitive.
