bear in mind that they've been talking about weekend parking restrictions around Shepherd's Biush to control the crowds coming to the new Westfield Mall. I don't know if these will extend as far as Olympia.
You can get there on the tube - a special service will run from Earl's Court, I should think, and probably High St Kensington.
you can pre book parking at Olympia/Earls Court, its pricey but at least you can put all your goodies straight in the car [ and go back for more] plus it saves parking up and using public transport, weighing up the pros & cons I would and have[ for thr Ideal Home Show] pre booked their car park, but you wont get in on the day without booking.
Have a great time.