I very much doubt that Blair will enter the Iraq Inquiry via the front door. He can get in to the QE2 Conference Centre from the back by passing through the Institute of Mechanical Engineers building. That seems pretty likely. A strong detachment armed with buckets of blood should watch that route.
Or he can arrive by an underground route using the spur to the QE2 conference centre from the old tunnel that connected Bomber Command (now known as The Citadel bunker) in Marsham St to the Cabinet Office and the MOD. As this tunnel network is an official secret I doubt they will want to risk him appearing mysteriously from nowhere, though.