LONDON — The British government announced Sunday that it would bring Northern Rock, the struggling mortgage lender, under its control. It was the first nationalization of a bank in more than a decade and a huge blow for the government of Prime Minister Gordon Brown. The government rejected two takeover proposals for the lender, which ran into trouble last year because of a funding shortage that followed the subprime mortgage crisis in the United States. The government was forced to shore up the company with about £55 billion, or $107 billion, in loans and guarantees. “The government has completed its...