Ratings agency's reminder of 'longer-term issues facing the UK' destabilises foreign exchange marketsThe pound fell sharply today after ratings agency Fitch warned that Britain was "potentially most at risk" of losing its triple-A sovereign debt rating.Britain's ballooning budget deficit has caused similar warnings from other ratings agencies and Fitch said it expected the British government to soon articulate stronger plans to reduce the deficit.But nervy foreign exchange markets did not like the reminder of Britain's deficit, which is heading for a record of 12.5% of national income in the current fiscal year.Sterling, having hit a three-month high against the dollar on Monday, fell back to around $1.66 against the dollar, although it later recovered somewhat after Gordon Brown stressed that ...