I would also drive, its preety straight forward... M25, M11, A406 ( North Circular ) A13 - Might hit abit of traffic as you join the north circ, but at that time, it should be decent.. Alternativly, you could go abit longer round the m25 and get on the a13 from there.
If you want to train it..get into london..get on the DLR and Head for Custom House!
P.s Drove to Excel a few months back..Easiest place to find ever..and parking is 'reasonable'..
Google maps sais quickest way is, down the m1, and straight on the north circ..I would highly recommend, NOT going that way. Top of North Circular is the WORST!