If it were me, I'd leave the M25 and get on to the A13. I'd take the exit at Prince Regents Lane (A112) and park on one off the streets around there. If you look on google maps, pretty much the whole area from Canning Town to Beckton is residential. It's pretty safe to leave your car around that...
OK this is a long shot. I live down in London and need two tickets for the West Ham replay on Tuesday. I noticed earlier that 500 more tickets have been issued for this game but it would seem that they can only be bought from the box office at Goodison. Are there any good samaritans on this site...