Problem is that either that results in quite short sets with one class 43 (4 or 5 coaches) or you end up running with two locos which can pull more than the 6 or 7 coaches in the rake.
Also, unless you are going to spend a lot on the Mark 3's, there is only a short time frame until the DDR kick in.
I wouldn't be surprised if we end up seeing a DMU order in CP5, even though one isn't planned, just because passenger demand from growth requires it and we can't electrify fast enough. Although such an order may mean that the planned order for CP6 is skipped or at the very least is later on rather than the first units arriving on day one of CP6 so the politicians can claim that they kept to the timetable and didn't order any in CP5.