Finally, just as a political point: Blair was popular within his own Party, his own Party held a pretty significant majority, and unsurprisingly these two factors meant that his own Party supported him on this matter (except Corbyn and a few others, who spoke out against the decision to go to War). Under all those circumstances, it's extremely difficult to hold such leaders to account -- they're just too protected, politically.
It seems to my mind preferable to be in a situation where the (former) leader of a Party that holds a majority in Parliament is not, for that very reason, protected from investigations into their conduct. To describe it as regrettable that Blair's potentially having knowingly misled Parliament wasn't investigated is an understatement. But, again, what does this make your point? That we should follow that example, and never investigate Prime Ministers for lying to the House, no matter over how small the matter is? Clearly not. We should surely hold our leaders to the highest standards of conduct. Blair got away with one. The lesson there is surely *never* to take potentially having lied to the House lightly.