Hi there John001. Did you specifically want a venue adjacent to the M40, so you can stop for a while get a photo then carry on your journey?
If this is the case then the A40 runs parallel to the M40 between junctions 7-5 southbound. Personally I often exit at junction 6 and drive through stokenchurch to west wycombe trying to get photo's there are ample parking opportunities.
If you want to get high as to have a view over Kites as they soar below you i can recommend going to the car park for St Lawrence church. Pass Hellfire caves on your left and continue up the hill and park at the top.
There's a map here
Or Coombe Hill
Another link here for you
The former you don't even have to exit the car to get great views. The latter is a short level walk from the car park to the monument with stunning views over the vale of Aylesbury.