David Cameron, the British prime minister, has sharply criticized banks and building societies for its restrictive lending practices. Cameron said lenders have gone “too far” to weed out poor borrowers, preventing “good risk” home buyers from securing mortgages. In a related development, Hector Sants, head of the Financial Services Authority, has called for explanation of the agency’s proposed new mortgage rules, which ministers say will make further impeded the housing market and stifle recovery.