Have you tried AA Route Planner ?
One thing I don't understand is why you would need to drive there ? I mean I can appreciate that you have paid for your car parking, which I presume its non refundable ? , how ever is there another reason ?
Why I ask is if you really cant drive there why not get a train or the tube into London ? What I mean by that is drive to either your nearest Train station to London or your nearest Tube station. I'm not sure what direction your coming from so cant say what your nearest one is how ever I live down on the South Coast and when myself and my family spend the day in London we drive to our nearest Tube station which is Morden and tube it into London. Morden is a great place to park as there's a really good car park less than 5 mins walk away from the Tube station in fact you could almost say the car park is on top of the station ! The only down side is the fact that it could take any where from 3/4 of a hour to a hour to get into London but as long as you factor that in your journey time etc you shouldn't have to worry. If you think that Morden may be something you may look into doing its really easy to get to Kensington Olyp as Morden is at the end of the Northern line so catch any tube going and change at Clapham North for Clapham High Street ( its a easy walk between the Stations ) and then catch a Overground Train to Kensington Olympia :)
Just go to TFL web site and check out times etc
Just a idea
I will be driving there because it is cheaper to drive and park than catch the train and about an hour quicker. It's also a lot harder to carry everything around rather than leaving it in the car until its needed.
I know what you mean, last year I drove early, parked at Olympia and walked straight in, a lot easier as in cosplay to. This cycle event is a real pain.