If you leave LHR open, then you need to regulate massively.
Otherwise BA would simply stay and help themselves to the current traffic that currently fly with their competitors.
I live in London, I know public transport well. Making the economic driver of the M4 corridor head for the East Coast is a politicians fantasy. So if BA stays in West London, who else is "allowed" to stay? Let the fun begin!
Incidentally makes T2 and T5 all a bit of a waste of time if this was serious.....