Any government will have a struggle to get their program through parliament. As you've suggested, any bill has to be ratified by parliament, that includes representatives of all parties. My point is that at this moment in time, based on current opinion polls and voting projections, the next parliament could be made up of parties that obtained less than 50% of the vote. For example, the Lib Dems are actually expected to retain the majority of their seats, despite seeing a huge drop in voter support. That's because their vote is concentrated in certain areas of the country allowing them to win seats and remain the 3rd biggest parliamentary party, when they are predicted to be the 5th biggest party nationally.
Proportional representation is in my opnion more democratic, more representative and leads to more progressive and less agenda driven government.