(Reuters) - London's mid-cap index outperformed its European counterparts on Friday after the Brexit Party lent further clarity ahead of the Dec. 12 election, while hopes that a U.S.-China may be imminent helped the FTSE 100 eke out gains.The FTSE 250 advanced 0.9% as domestically-focused stocks rose after Nigel Farage's party stood down from more seats not held by the Conservative Party, which could help Tories gain a majority in the upcoming election.As a result, the index bagged its third..