Surely got to add to that other 15x units at other TOCs (EMR, GWR and SWR), not just the ones at Northern? Makes little sense replacing the Northern ones and none of the others - beneficial to replace them all in one go.
Northern is government owned and in absence of Great British Railways it is being used to run the tender but a large number of units would go to replace sprinters used by other ToCs. 450 units is bigger than Northern's entire fleet! The delivery time is quite long too. I think the intention is to provide a moderate amount of work each year for 5-7 years. This enables CAF and Siemens to tender with their UK factories. Social benefit clauses should make UK production a near necessity. CAF Newport is small and Siemens new factory in Google will need most of its production capacity for the underground contract.