As I recall, the special Cabinet team that Cameron set up to consider the Davies report was criticised for not including those opposed to LHR expansion.
Did that group ever report to the full Cabinet?
If either Boris or May is elected, and their not being involved in the original meetings, I suspect they will insist on giving the matter very careful consideration which could take weeks, months? And with so many other priorities .......
Seriously though, whatever the arguments for LHR, the case for LGW post BREXIT must be tempting if it doesn't involve money from the public purse.