Have to agree with Terry there. As I understand it, the highest charges will be for the busiest roads at the busiest times. Somehow I don't imagine people will be towing their caravans around the M25 at 8.30 on a weekday morning, so I can't see it having much of an impact.
For me the travel costs are the least of the costs of going to festivals - the tickets, the beer, the food and the CDs all add up to a heck of a lot more. I honestly don't know how so many people can afford to go to the number of festivals they do!