The "defacto two party system" results from the first-past-the-post voting system, not the number of voters who cast their ballot for other parties.

Other parties got around 30% of the vote at the last General Election, but won only 13% of seats.

(It is also worth noting that the Liberal Democrats did not gain a "significant increase in votes" in 2010 (23%) vs. 2005 (22%), and that they won fewer seats in 2010 than in 2005.).