The two most important central banks in Europe chose Thursday not to change their interest rates, despite recent suggestions that some monetary authorities might re-double efforts to stimulate economic activity. In Frankfurt, the European Central Bank left its benchmark interest unchanged at 1%, while the Bank of England kept its key rate at 0.5% and decided not to expand its debt purchasing program for now. In Britain, there have been signs that some policy makers would consider a new round of debt purchases to revive an economic recovery that had lost steam. Many economists expected that support for expanding the program to buy mainly government debt, which was halted at £200 billion, or $317 billion, almost a year ago, would gain momentum from this week’s committee meeting at the bank.