Let's face it. The commercial case is for Heathrow.
If you build a second runway at Gatwick, all you get is a replica of the existing Heathrow - which is too small (hence the need to expand).
What the market wants (to be able to compete with EHAM, LFPG, EDDF etc.), is a much larger EGLL. A decision in favour of EGKK would be a totally political fudge -& leave both 'LL & ' KK being too small to compete with the other airports.
End of story.
The 'LL option is what it should get to meet the requirement. You can expand EGKK, if you like, & it might well be an improvement over existing facilities, but the general requirement needing to be fulfilled is Heathrow !