Well you made the claim, but
this shows that the Tories were likely to have achieved majorities if past elections had been AV. Add to that the fact that a large proportion of those LD voters that would give their 2nd preference to Labour before last year are now likely to vote Labour first I'd say that AV is at least as likely to benefit the Tories as Labour.
What a bizarre point, the centre-left hasn't managed to get 50%+ of the vote for donkeys years either (or, depending of what definitions you're using, ever?) and the Labour vote at recent general elections has been in a similar range.