Not sure where the class thing has come in here. Although on another tack, casually linking knife crime and the cookout is more than slightly dodgy.
As to the benefits to Forest Hill? Well, for sure I know that the road between the station and Horniman was constantly full of people, so if only a few popped in to any of the shops there, that would be a massive benefit to Forest Hill in itself. I'm afraid I don't have individual figures of sales increases. I'd be amazed if business was down, though, wouldn't you?
Importantly, as an event increasing in popularity, it acts as great PR for Forest Hill. And I think Forest Hill needs a bit of good PR.
And yes, can't people be happy with having 364 days of the year with full car use, and one day a year where they might be slightly inconvenienced and have to use the train or come back a bit later in the day, if that means 10,000 people get pleasure from coming to Forest Hill? Most of this is all just real nimbyism cos it puts people out momentarily. Read the Sundays, get the train to somewhere nice. Or...now here's a thought...go to the cookout and have a nice afternoon tasting stuff you wouldn't normally taste and listening to music you wouldn't normally listen to.