I think most people will be getting the train. Parking will be a pain, I think it is easier to get the train. Out of interest, when is everyone going down? I'm going down tuesday, is anyone else going that early?
Go to www.southernrailway.com and check the train prices from there. I just booked mine going up on the 5th at 13:42 and coming back on the 20:10 on Sunday for the princely sum of £7.50 (from Clapham Junction, I think it might be the same from Victoria though.) Probably cheaper than petrol and quicker than driving!