I've done Stanmore before and is fine. You do have to be careful parking around the residential area. I've seen people get towed when in wrong place.
Green Man pub, right above Wembley is good. On the Hilton Hotel side. They charge a bit less than the official car park and there is a lot of room outside for beers and food. They usually get a bottle bar going so you don't have to queue inside. Have been doing barbi as well recently.
......only down side is it is a VERY big hill to treck back up to get to your car.
Just do the official car park now as my dad doesn't like the walk. Think it's about £27 to park but you are feet from the entrance and it's a good get away too.