Minority Parties - Election 2010

There is a lot of talk about minority parties wielding undue influence on Government under an electoral system of Proportional Representation.

Both Thatcher and Blair governments never got more than 40% of the vote, but ran the government without partners. It is worth asking: isn't this wielding undue influence on Government?

Furthermore, why is that conservatives (small 'c' intended) always oppose any progressive policy before accommodating to it?