Shares closed lower on London's Stock Exchange Wednesday as investors assessed the news that Prime Minister Margaret Thatcher is being challenged as leader of the governing Conservative Party. Former Defense Secretary Michael Heseltine said he will challenge Mrs. Thatcher in a ballot among Conservative legislators Tuesday. The Financial Times-Stock Exchange 100-share index fell 10 points, or 0.5 percent, to close at 2,046.0. The Financial Times 300-share index was down 10.4 points at 1,583.2. The Financial Times 500-share index fell 5.88 points to close at 1,086.64. Volume was 356.5 million shares, off from 422.9 million shares Tuesday.