I can't find any reference to this, other than that the long term deployment of the IEP may be different to the long term business case so the bidders will need to determine how best to optimize fleet deployment. Is it not planned for the IEP deal to be signed before the franchise is awarded? If so then it would be a bit late for bidders to propose alternatives.
The franchise consultation draft said the exact opposite to
gwr4090's point, that bidders for GW had to use IEP.
There was a prequalification questionaire query about Thameslink/GN regarding IEP that led to the answer from DfT that other units could be chosen by bidders. Problem was that all the clarification bulletins included answers for all three franchise processes mixed in together - which may have resulted in misreading the position with IEP on GW.
