It’s an interesting example, by the way, of just how skewed the British electoral system is against the Tories. If I’m right, a 3 point difference in the national polls leaves them almost 200 seats behind Labour.
The British electoral system isn’t skewed against the Tories—at least, not any more, as Scotland’s overrepresentation in Parliament has finally been done away with; it’s skewed in favor of whoever wins the plurality of the national vote. It’s almost (but not exactly) the exact same effect as we see in the U.S. electoral college: “landslides” in the electoral college are easily manufactured by relatively small differences in the popular vote.
The effect is also a partial consequence of Britain’s nonpartisan redistricting system; gerrymandering in the U.S. depresses the number of districts that are likely to “swing” from one party to the other, while the British process tends to produce a larger number of districts close to parity. (However, unlike post-Wesberry America, there is no requirement of strict population equality for constituencies in Britain.)
Shugart and Taagepera’s Seats and Votes explains things far better than I can, if you can find a copy.