Maybe they never wanted to build these spiritual structures in populated areas, which London would have been at that time. Stonehenge is still in the middle of nowhere and was specifically built on a in a very romote location.
Are you sure about the above? From my understanding the opposite is true. Salisbury Plain was extensevely cleared and farmed in the Meso/Neolithic Ages because of the easily worked soil in contrast to the heavy clays of the London area.