"and if the public decide they want a tunnel (say to further the existing limited knowledge about Silbury)?"
The tunnel isn't being proposed to further existing knowledge about Silbury, but to remove the voids above the 1849 and Atkinson tunnels, take new measurements and readings from the turf stack as Atkinson cocked this up, and in addition whilst they are there retrieve some of the previously untouched petrified prehistoric ground material.