I was also going to suggest Liverpool Street as an option. But not many people will walk to Tottenham Hale (it's a hike of 25 minutes or so) so the Victoria line to Euston will be quick and easy. Then either bus / taxi to Paddington for HEX/Elizabeth. Or Northern to Tottenham Court Road and Elizabeth. Changing at T2-T3 if necessary. Basically you want to be a bit counter intuitive to avoid the crowds. If you see a 133 bus or similar, servce to Walthamstow, after leaving the stadium and the traffic is flowing then do that, getting off at Blackhorse Lane.