from what I've heard from those who've been, on the actual pitch itself.
please read this parking information from the stadium's website, there are various places you can park:
It is recommended that any visitors wishing to drive to the stadium only use the Official Wembley Stadium Car Parks. To reserve your space, please visit www.csparking.com/stadium To avoid disappointment, please pre-book your space early.
A protective parking scheme operates in the local residential streets on Stadium event days. Only vehicles with the appropriate resident or business event day permits will be allowed to park in the area. The local council has stated that illegally parked vehicles will be issued with large parking fines and towed away. For more information visit Brent Council's Information on Event Day Parking click here
Note: there are a number of unauthorised car parking operators within the Industrial Estate on the approach to the Wembley complex.
it seems you are late on getting in on the official parking, I would use google maps or a GPS to look at the stadium's vacinity for additional places to park. even if it might be a bit far from the stadium.