I am not certain,but I believe that LHR slots are allocated,but not sold.
The selling occurs between airlines,as ALL the slots are currently allocated,and there aren't any spare.
If there is to be open skies,then I'm sure there would have to be a total reallocation of slots,where EVERY slot would have to be applied for(meaning ones currently held ),and some system of fair allocation worked out. Quite how this would be done is hard to see,but to give all applicants a bite of the cherry,clearly BA would suffer enormously.