Tell me which General Election had any realistic prospect of any party winning other than the main two? There might be more *options* but that doesn't equate to more *choice* if the other options have no prospect of winning. The last time a party other than Labour or Conservative had any chance of winning was in 1918 when the liberal party was at the end of its decline and replacement by the Labour Party which had only started 18 years previously. 1918 also coincided with the vote being extended to all men over 21 and most women over 30.
Since you ask, 1931,1935, 1955 (if we are rounding), 2010 (coalition). Generally the winner has over 40% of the votes. 50% isn't really relevant as you have to take into account parties in NI and the SNP.