Southeastern are further ahead than Northern. Southeastern have already submitted their business case, Northern from public tenders wouldn't be in a place to do that. Their depot upgrade feasibility works aren't completed and would be one of the prices needed for their business case.
To be fair Northern and GWRs Project Churchward also had outline business cases. Although the timeline has slipped so far that they could be deemed out of date.
Although conversely, the case for doing nothing and just patching up the old trains (that are now 34-41 years old) indefinitely, is looking more like throwing good money at a bad job, every week that it continues.
However I am very wary of the political appetite for authorising new trains for the South East, without also authorising new trains in the North. Seems to go against all the levelling up dictats.
I do wonder if politically it will be easier to sanction 2 or 3 regions orders (even if each goes to different manufacturer) than just approve a South Eastern order (which has been suggested will happen late 2026)
My hunch is the Lord Hendy rolling stock review report will be timed to come out, so that do nothing looks bad, and multiple orders can be placed. And that has the advantage of stopping legal challenges if everyone is getting a share.