The website makes reference to designated event car parks, so you might be OK:
http://www.thecityofg...ps/travel-information
I'm thinking of going to the family day at Notting Hill Carnival tomorrow but I've just discovered that my normal 2 trains into Liverpool Street have been replaced by Bus>Train>Bus>Tube and I can't even get to Liverpool Street before half past one!
However I've also worked out that I can go the long way round (via Cambridge) with a bus and two trains, getting into London a full two hours earlier, so I might still go. (I'll have to see what time I wake up tomorrow!)