It'll be rammed wherever you are around Wembley. Have a look online to find a car park you can pre-book. The stadium is right next to an industrial estate where most of the buildings offer parking. Not too expensive IIRC. Final tip is to try to leave before everyone else - watch the last song from the steps/entrance into your section of the arena and then hot foot it out and to your car as quickly as you can. It's a nightmare trying to get out at the same time as everyone else - total gridlock!
I went last week for an England game........never pre booked, loads of indian/Arabic men standing infront of industrial estate factories selling space in the car park, they give you a ticket and you can park there, I paid £10