"The question is how long the U.K. can hold on to its AAA status," said James Knightley, economist at ING Bank in London. "With the U.S. and France having been downgraded by one ratings agency in the past couple of years, another disappointing U.K. borrowing number and a widely expected contraction in fourth-quarter gross domestic product on Friday will intensify the threat of the U.K. suffering the same fate."