A bit of a shame that Khandro seems to have not yet acknowledged my criticism of the UK system because it screwed UKIP in 2015.
I should also add that spicerack is essentially correct about the election of 1997 -- it wasn't quite a 2-to-1 advantage for Labour but was near as dammit. In 1983 (another disgracefully disproportionate election, the SDP/Libs scraping 20 seats from 25% of the popular vote), the Tories had "only" a 1.2-to-1 advantage over Labour.
It's also a turnout issue: Labour has a tonne of seats that it comfortably wins on pathetically low, c.50% turnouts (and the Tories have the opposite problem).