I think the barge into Cameron WAS deliberate and highlighted the lack of attention by his CPOs.
Simon Jenkins of The Guardian summed it up pretty well::
The Met police’s famously overstaffed and underworked VIP protection mafia were sunning themselves on the pavement when a man charges towards the prime minister. Don’t worry, they instinctively think, his beard, his dreadlocks, his T-shirt with upside-down cross clearly suggest a harmless Yorkshireman late for his personal trainer. Only when he is within stabbing distance of Cameron do they go berserk. How will it look on CCTV? They have just protested against cuts in the VIP protection budget, and now look at the mess. They cannot spot a potential threat at five paces.