Cut the bullshit for Christ's sake!
"he and other group members planned to pull out megaphones as the royal procession approached."
I think we can all assume these won't be heartfelt congratulations. And you call this entertainment? That's asking for trouble, plain and simple. You don't put a fox in the henhouse, watch the carnage ensue and say: "Wow, that was surprising and totally unexpected."
"He was arrested on suspicion of conspiracy to cause public nuisance."
So to answer your question on can someone be arrested of an offence without committing said offence is yes. I know you've danced around the issue of attempted murder and conspiracy to commit terrorism etc. by the logic that "They are more naughty" but that's not the point. The severity is not the issue here, public nuisance is a crime. The police had reason to believe some bad shit was going down and felt they had to sort it out. Deal with it.
"More than 60 people arrested in recent demonstrations have already been banned from central London on Friday as part of their bail conditions."
Man, these likeminded individuals sure sound like swell guys.