Here are quotes from the instructions for the London anti-extradition demo 
that were posted by Janis McKinnon.

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     Apparently the police have said that there are no plackards 
    allowed and everyone is to be dressed smartly. 
    ...

     IMPORTANT: 
     Once the demonstration crosses Saint James's Park it enters
     the Designated Area around Parliament Square and section 132
     of the Serious Organised crime and Police Act 2005 comes into
     play. 

     Remember that the shortest route from St Jams's Park to Marsham
      Street actually passes right outside New Scotland Yard Met
     Police HQ. Any breach of even the newly imposed Conditions is
     also a criminal offence. 
     ...

     The  march / demo,protesting against the one sided extradition
     treaty, apparently  having been automatically granted
     permission, is also automatically subjected to arbitrary
     Conditions. 
     These conditions can be added to or changed at a
     moment's notice by the senior police officer on the spot, who
     will definately be watching. 


     Even the "dressed in business dress, anti extradition demo"
     could have arrbitrary Conditions slapped on it at a moments
     notice. 
     e.g. most of the demonstrators could be held back from
     approaching the Home Office building itself (not neccessarilly
     for security reasons, but possibly to hide the number of
     supporters from the waiting media pack) and only a couple of
     people let inside to deliver the letter. 
     Even if you turn up as a one person demonstration, that is also
      covered by this draconian law. 

     See http://ParliamentProtest.org.uk