The whole thing seems to be wrapped up in excessive concern by the UK government wanting to have a Fixed Price contract without an adequate design. This was what caused the mainstream UK contractors to all walk away in the first place, because someone in the UK govt had it in for them after the experience of building the Falklands airport, which was done Cost Plus. I suppose we just add this one to the long litany of procurement failures the government has got into over time. Professionals such as Boeing (see above) were ignored because it was felt they were just wanting things done to be able to charge the project budget for them.
There was a belief that the major UK contractors (Balfour Beatty, Carillion, etc) would just take the money and subcontract the physical construction out to the likes of Basil Read anyway. Sorry, there's more to it than that, they like to do complete and working projects that can be examples for future clients. If there were these documented concerns around they would have wanted to get those understood and fully handled at the start.