UK Campsite shows several some distance away, nothing really close. You may be best looking at train services to the stadium and finding a site close to a station on an appropriate line.
If there is a risk of going into the Low Emission Zone (LEZ) check that your van is compliant first.
Hi and welcome to the Practical Motorhome forum – thanks for posting.
There's Lee Valley Camping and Caravan Park from where you can get a Tube into Wembley. Other good London sites include Abbey Wood and Crystal Palace, both Caravan Club sites, but both south of the river – although you can still get to Wembley via public transport from there. Hope that helps!
YES The LEZ is within the M25. wembley Station for the Stadium, is on the Jubilee line. or the Metropolitan. So look for sites within distance of those trains.
IE Stanmore end of Jubilee line. Some where along Amersham/Chorleywood area, maybe.