What gives the police the right to arrest a person who has not committed a crime and wasn't planning to commit any crime?

Yesterday, long-time activist and professional thorn-in-the-side-of-the-establishment Chris Knight was pre-emptively arrested by the police (video below).

The crime? Planning a 'zombie wedding' stunt that featured a mock execution of Prince Andrew with a guillotine. Not the nicest of stunts, I'll grant you, but an arrestable offence?

Keep in mind that a Channel 4 film crew were with the trio who got arrested, filming for the 'Unofficial Royal Wedding' Some of their equipment was also confiscated.

The 'Zombie Wedding' was billed as a "right royal orgy" with "rumpy pumpy and guillotines" and explicitly stated, "this is a totally non-terrorist event". What did the police say? This:
This evening, 28 April, officers arrested three people - two males aged 68 and 45, and a 60-year-old woman - in Wickham Road, SE4 on suspicion of conspiracy to cause public nuisance and breach of the peace.
Unbelievable. Watch the video:

The police had already "made it clear" in advance that:
This is a day of celebration, joy and pageantry for Great Britain. Any criminals attempting to disrupt it - be that in the guise of protest or otherwise - will be met by a robust, decisive, flexible and proportionate policing response.
- Cmdr Christine Jones, Metropolitan Police
Earlier in the day, the same strategy was applied to raids on various squats in Brighton:
During the attack on Grow Heathrow, sleeping people were dragged out of their beds by up to 40 cops. The police left having seen mostly vegetables and some bees. I'm unconvinced that vegetables (organic or not) required the possibly illegal presence of dozens of riot cops to stop them from being repurposed as guillotines or whatever mad nonsense the cops will inevitably say they had specific intelligence about in the press release which will be sent out just in time for the evening news.
Guy Aitchison is right when he says this is just blanket, political policing.
The political intent is quite clear. The police have gone far beyond their role as upholders of the law. At vast expense to the taxpayer, they have gone out of their way to ensure that tomorrow's event delivers exactly what it says on the tin: a huge display of ostentation and state power used to cement people's loyalty to the establishment at a time when your average citizen - sorry - subject, is getting screwed
But this isn't just about the Royal Wedding is it? The police are arresting activists pre-emptively, without any evidence that a crime was going to be committed, just to restrict political protest.

Where are the checks and balances on their power? Where are the right-wing defenders of civil liberties now?