Nice idea with the shadow but there are 2 main problems apart from not having much sunshine in the UK:
1. The length of your shadow changes with the angle of the sun so it will only work if the sun is directly overhead (look at the long shadows cast near sunset)
2. As Baston has said, you need to be able to see directly under the aircraft to see the shadow in this case - not very likely in most aircraft and certainly not to the point where you could see the whole fuselage, let alone the rotor disc.
Back to picking good markers and a careful recce methinks.