Did people notice a 'Radical workers bloc' is meeting at Kennington park - mostly solfed and afed I think, so that will be pretty black blocish I imagine, if you want that kind of fun.
Given that the TUC has been getting stewards trained by the Met I think these 'unofficial' feeder marchers are great - and preferably wouldn't meet up with main march at all. Some people say it should meet the main march to increase the numbers there but why? It's not a numbers game. It's not a question of who has the most 'moral authority'. The govt won't have an official counter going "Oh right, now it's over 200,000 we have to take notice."
For me there is as yet no serious way to oppose govt power, no organised movement you would recognise as such, so I think disruption is the best we can do, to demonstrate a refusal to play their sordid little game, and to start building networks of people who understand the nature of the fight. And in terms of this demonstration, that means it is less about numbers and more about a bit of planning and a bit of determination among groups.