Exactly, the specification didn't specify loco hauled.The DfT wanted 442s as a push pull unit loco hauled. It’s well known.
First Group ideally wanted all 802s but Hitachi couldn’t deliver by 2019. They then went with brand new Mk5a stock.
I'd be concerned about the weight of the 68s limiting their speeds, Norwich has some SP speeds around it.I would suspect that the MK5a coaches would be better doing a route like Norwich to Liverpool Lime Street,
The best for Norwich to Liverpool is to split it at Nottingham, run spare 755s on Norwich to Nottingham and double up the 158s on Nottingham to Liverpool.