Impeaching Tony Blair is, as a practical matter, impossible. It’s not going to happen.
But putting that aside, would it be legal?
Iain Murray says no, citing Britain’s Human Rights Act, which provides for trial by an impartial tribunal.
Eugene Volokh says yes: since, he says, the penalty faced by an impeached official is only the loss of office, an impeachment needn’t be subject to the procedural rules that come into play when the accused is threatened with the loss of life, liberty, or property.
Perhaps I’m missing a subtlety here, but it seems to me that Eugene is being taken in by the American sense of the word “impeachment.”
Under the Constitution, impeachment applies only to officials, and the maximum penalty is dismissal from office and disqualification from future office-holding. So that’s what “impeachment” means to us.
But those rules reflect a decision by the Framers to deny to the Congress the unlimited judicial power long claimed by Parliament: not limited to officials, and not limited in the sentence that could be imposed.
In principle, if Blair were convicted on impeachment, he would be liable to any penalty the House of Lords could devise. Impeachment in the British system differs from a bill of attainder only in providing a trial. (Strafford was executed under a Bill of Attainder, but only because he defended himself so well against a capital impeachment that the Lords wouldn’t vote to convict.)
On the other hand, if a majority of the House of Commons wants to get rid of Tony Blair, no impeachment formalities are needed: by custom with the force of law, if the Commons pass a motion of No Confidence, the Prime Minister must either resign or call new elections.
So for the legitimate purpose of getting rid of a PM, impeachment is superfluous (as it would not be in this country, where Executive Branch officials hold their tenure independent of the legislature). Any other purpose is (I think Eugene and I agree) illegitimate.
Update: Eugene suggests that the sponsors of the Blair impeachment movement don’t intend any penalty beyond dismissal from office. But in that case, why bother with impeachment when a no-confidence vote works just as well? Perhaps the goal of making the suggestion is to imply that Blair’s actions were not merely wrong but criminal. Bad manners, say I.