I'll agree with your assessment of the holes in the security for the H of C, but that's as far as it goes. IIRC they used condoms filled with this purple powder. One of them hit PM Blair squarely on the back. Whether or not the powder was harmless is irrelevent. Their actions were not non-violent. These tactics diminish their credibility, regardless of what cause they were championing or point they are trying to get across.
I'd expect this out of the left. This is not the manner in which adults deal with disagreements and conflict in a civilized world. And it is still unacceptable.
Prairie
As for poor old Tony getting hit on the back by a johnny full of purple flour, apparently he didn't even feel it and he certainly wasn't in danger of being harmed by it. British politicians are more used to a bit of "aggro" than their US counterparts seemingly.