OshkoshGeorge is right, at least according to the documentation, british, that I read concerning the issue. Slots are not sold, but allocated, however, airline with LHR slots do trade them.
If a slot is forfeited it goes to the pool, to then be handed out to eligible airlines, actually those that have put in for them.
I doubt you will find many US pilots who support this, since I am still at loss to what the US carriers gains, just like someone fails to see what british air carriers gains. So, why push for it, why does BA want it? What airports are they restricted from serving in the states.
However, I will still maintain, that slots will have to be provided at LHR for the US carriers that wish for them. LGW or STN might be nice, but the businessman prefers LHR for access to London and for onward connections.