I am advised in an email from Professor Robert Stevens, author of Torts and Rights, that even George Monbiot's symbolic mode of citizen's 'arrest' is not entirely unproblematic from a legal point of view:
What Monbiot is urging would still be a tort. Even though he is not suggesting imprisoning Blair, what he is suggesting would be a tortious battery, as it is an intentional unauthorised touching without consent. (In many day-to-day touchings - e.g. tapping someone on the shoulder to get their attention - there is implicit consent, but not with regard to what Monbiot suggests, as Blair would obviously not consent to it.)
Amusingly, if someone did act in this way as a result of Monbiot's urgings, Monbiot would also be liable, as he would have procured the wrong and the wrongdoer's actions would also be attributed to him. I would suggest, as well, that his employer, the Guardian, would be vicariously liable for Monbiot's wrongdoing.