LONDON—There was a time, as Mr. Banks put it in "Mary Poppins," when the British pound was the admiration of the world.

It wasn't in the first quarter of 2013.

Sterling has taken a remarkable nose dive. It started the year at $1.6246 and closed the quarter down 6.4% at $1.5202, after falling below $1.50 for the first time since 2010....