Why would Ferrovial, a massive construction company, not be in favour of building a new airport? Even if they only built one runway and one terminal, it would net them many millions.
It isn't just the fact that they have already invested billions into LHR, which they own, it is a simple question of probability.
With FBI, they would be tendering out with several other contractors, and they may also be just part of a conglomeration of contractors to do one particular part of the job - although more likely all the foundations, all of the runway or the terminal complex, rather than a bit of either.
Why would you want to be in favour of something you might get a partial benefit from when its very existence would trash a very valuable asset that is all yours?
And today Boris has apparently denied it strongly! does he read PPRuNe?
Hardly - bookies have had Boris as favourite for next Tory leader for several years now, despite him being not even in the cabinet, but not an MP either. Therefore every time a possible seat comes up for him to take we get the same speculation. The other oft-speculated option is that BoJo's brother Jo yields his Orpington seat for him.