Margaret thatcher never got more than 42% of the vote.
The split of the left into two different parties - labour and the sdp - kept thatcher in power for so long. If you add the total votes from the sdp and labour in constituencies north of watford gap, thatcher would have the same number of tory mps as john major did in 1992.
Actually, in 1979 she won 44%, in 83, 43% and in 87, 42.5%. Also, why assume that in the event of there being no Alliance all their voters would have gone Labour? If you look at all polling evidence it suggests about a third of Liberal, SNP and SDP voters lean to the right wing. In a 2 party system it still would have been 56-44 to Thatcher in 83 and 87.
In a 2 party system Thatcher would almost certainly never have become Tory leader, and would certainly not have been able to push her agenda through.
