LONDON/NEW YORK—British bank Barclays faces an estimated 450 million pound ($592 million) loss and regulatory scrutiny for exceeding a U.S. limit on sales of structured products, some of which have surged in popularity since Russia’s invasion of Ukraine. Barclays also said on Monday that it will have to delay a planned 1 billion pound share buyback as of the loss, which it will have to incur as a result of buying back the securities in question at their original purchase price. Shares in Barclays closed down 4 percent after the bank said it had oversold billions of pounds worth of the securities over a period of about a year, overshooting a $20.8 billion limit agreed with United States regulators by $15.2 billion. The major regulatory blunder is an early embarrassment for C.S. Venkatakrishnan, the newly-appointed chief executive of Barclays, whose previous roles included heading up the bank’s global markets and …