Despite being a staunch opponent of it, I don't think Thatcher killed it per se, I think it'd be more adequate to say it killed itself.
Note, for example, the dismal failure of Labour leaders like Niel Kinnock and, to a greater extent, Michael Foot - staunch socialists who were chewed up and spit out by the electorate. The only real left-wing alternative is the Liberal Democrats, and though they've been on the rise in recent years
I think the main reason for this is the absolutely disastrous economic situation in the 70's - directly a result of social-democrat economics, which could only sustain itself on the post-war boom. Which ended in '73. Plus people weren't too happy with men like Arthur Scargill holding the country to ransom.
However, although the pendulumn's mainly swung toward more right-wing/centre-right government at the moment due to the legacy of Thatcher, this is much the same way it
swung toward the left in the aftermath of Attlee's legacy. It's always possible it could swing back again...
Read Cosgrave's 'The Strange Death of Socialist Britain'