In opposing the alternative vote, Britain’s Conservatives are trying to cheat history.
If you look at the start of every period of Tory-dominated government (i.e. 1931, 1951, 1970, 1979, 2010.), the percentage of the electorate voting Tory is always lower than on the previous occasion.
Today, the Tories can only hope to form a majority government by being the single largest party under a simple majoritarian electoral system. Under AV in most of those constituencies where the Liberal Democrats can get into the second place the Tory would be doomed. Parliaments are more likely to be hung.
AV is not about proportional representation (one person, one vote, one value) but merely a step to make the individual MP more representative of the voters in his or her constituency. But that is not to Tory advantage.