Not too long ago, costs at LHR were quite reasonable. However, around the time of the 2012 Olympics, very significant extra charges were brought in, including an extra "security" charge, probably with the direct intention of reducing the number of smaller aircraft wanting to use the airport. From what I remember (my regular pax was a fairly regular user of LHR back then), the lowest cost groups were removed from the scheme of charges and the least you would be charged was for something the size of a Boeing 737, so landing there to drop off/pick up one or two people in a light aircraft or small helicopter no longer made any economic sense whatsoever.