Did the Green Party cost Labour the election?

Polling before the election showed Green Party support coming mostly from the Liberal Democrats with slightly fewer from Labour and even fewer from the Conservatives.

Suppose that every Green voter came from Labour. If we reassign all of these Green votes to Labour then the Conservatives would have 320 seats, down 10 on the actual result and short by 6 of a majority. Labour would be on 244 seats, 12 more than what they actually won. UKIP would retain Clacton. The Liberal Democrats would lose Leeds North West, but hold on to their other 7 seats. We would almost certainly have a Conservative minority government relying on Ulster Unionist Party and Democratic Unionist Party support.

Even if Labour had successfully defended all 41 of their Scottish seats they would only have a total of 285 seats, very much short of the winning post.