Well, politicians will continue to say that they want something done/built until after they are dead - by saying it in their memoire! Of course, if those nasty capitalists won't spend the money? If those terrible Tory henchmen who run the big companies won't invest in what is, plainly, a jolly good idea? Well, then at least Boris has clean hands.
It's like Blair supporting the Olympic bid. If they failed, he would have been cheered for helping them. If they won, he would not be there when the financial pigeons come home to roost.