LONDON (Reuters) - The British pound slumped against the U.S. dollar and the euro in early European trading on Tuesday, following media reports that UK Prime Minister Boris Johnson was taking a hard line on Britain's transition period for leaving the European Union.Johnson will use the majority he won in Thursday's election to forbid any extension of the Brexit transition beyond 2020, leaving him less than 11 months to strike a comprehensive trade deal.The British currency fell 1.2% to $1.3155 ..