Shirley Williams

New party, then merger. A former Labour education secretary, Williams co-founded the SDP and in November 1981 won the Crosby by-election, overturning a Conservative majority of nearly 20,000 to become the party's first elected MP. She lost the seat in 1983, backed the merger with the Liberals and later led the Liberal Democrats in the Lords. Her stated reason for leaving Labour was its turn towards leaving Europe and unilateral disarmament.