From the Met's point of view it would be that their officers were reminded of their duty to allow lawful protest, whilst suggesting that the signs were there that this was potentially a big and (from their perspective) violent demonstration.
Williams gave a clunking reference to J18 (or "G18th" as he called it) when "protesters had run riot" and the City of London had been "badly damaged". In referring to the police intelligence aspect of the 27/3/9 briefing he concentrated on mentioning how police had managed to communicate with "a number of groups [likely to attend the G20 protests]", but not with "two groups, including the anarchists", a situation similar to that in the lead up to J18. The suggestion hanging in the air was clear.
As for drivers staying with their vehicles, this was something gone over in both Williams' and Harwood's evidence.
In Williams' words, "the role is to drive, to come when I tell you, to be prepared to pass out equipment, like shields, and to be available for food and drink. It is one of, majorly, security and safety of the vehicle."
He acknowledged there are times when a driver might need to leave the vehicle, or the area around the vehicle - toilet missions for one, or coming "to the aid of another officer, or a member of the public being attacked" - but he reiterated that "there are very few circumstances I can think of where the vehicle isn't the priority" for a driver.
Harwood is his evidence acknowledged these points. His given reasons for leaving his van were lengthy and convoluted...