My understanding is that if BAA choose to keep LHR, they must sell STN and Gatwick. If they sell LHR, they can keep the other two. Hence the reason why the CEO of BAA was using the argument over competition from abroad, not the UK. They still have 2 more months to influence the decision.
I would expect one of the Middle Eastern Sovereign Wealth Funds would buy one the London airports. Perhaps they would make an offer for LHR BAA can't refuse.