By definition, faith is not rational, but rationality does not mean knowledge is complete, it merely means it is determined by reason and evidence. The Catholic Church’s “Fides et Ratio” holds, inter alia, that rationality and faith are both essential, and therefore hardly mutually exclusive.
As a matter of constitution, the Prime Minster is correct, our constitutional church is Christian. The law developed in this country is also largely based on Christian doctrine so again he is right. What he is not saying is everyone or even the majority are Christians.
As for a Prime Minster talking about religion, he was addressing an event celebrating the 400th anniversary of the King James Bible – I imagine it was kind of expected.