(Reuters) - UK shares regained some poise on Tuesday, as massive stimulus measures announced by the U.S. Federal Reserve and others soothed nerves even as Britain went into a lockdown.The blue-chip FTSE 100 index rose 3.7% following strong gains in Asia, as investors took cheer from the Fed's offer to expand asset purchases by as much as needed to stabilize financial markets.Battered shares of Carnival Corp jumped 8% to the top of the index, while BP and Royal Dutch Shell gave the biggest..