I'm pretty certain that if they had been considered a viable solution, Arriva or whomever else would have found a way to convince DfT let get them in. The ITT may have specified that they must be ruled out, but if all the bidders had included them regardless do you think the DfT would have stood its ground? Nah, 30 years in public service and exposure to contracting tells me that the private sector is always better at getting its way than ministers and civil servants.
Well Arriva did manage to hoover up nearly all spare DMU's that were available at the time of the bid, I guess if those hadn't been available then I guess it would have ether been more CAF units or the 230's. Use of the 150's and 170's offers more flexibility than the 230's and now the 769 has been thrown into the mix.