I've been thinking about how I would do this. I think building new diesel trains is a mistake and whilst widespread electrification would be ideal, it is unrealistic in the timescale. I therefore (unsurprisingly) think that batteries are the way to go.
Since 158/9s clearly have plenty of life left, I'm going to ignore those, and concentrate on replacing the older sprinters and also the Castle class and Chiltern Mk3s. I'll also ignore Scotland, since they already have a plan, but otherwise take advantage of GBR which will presumably make it easier to move fleets around, or order similar trains for multiple TOCs.
Firstly, as others have suggested, I'd cascade 175s to Chiltern, to replace the mk3s, and also to free up around 50 carriages of 168s to become 170s. I'd also convert some electrostars to battery, in order to free up the 171s to become 170s.
Next, I'd order trains for short branch lines (up to 60 mins journey). 2x20m carriages, gangways, slow speed but good acceleration, basic design, able to charge on a vivarail type fast charge system, or using OHLE.
These would be used for branches in Thames Valley, Devon & Cornwall, Norfolk & Suffolk, Stourbridge, Coventry, Bedford - Bletchley, Salisbury 6, and quite possibly also in Northern and EMR.
That should replace all of GWRs 150s,whilst releasing enough 16x and 158s to also bin the Castles. Elsewhere, it would free up 755s for use on longer routes, and a few turbostars too.
Northern seem to have a lot of routes that spend a fair bit of time partly under the wires. Famously Windermere/Barrow - Manchester and ECML slows, but also things like Blackburn - Rochdale, Blackpool South, Rose Hill Marple, some services around Leeds and Newcastle, perhaps even Buxton, Southport-Manchester, and the CLC. For these routes, a new fleet of battery electric 331s would seem ideal, charging under the OHLE, as well as at some fast charging stations where needed.
These trains could also be used for short branches with fast charging similar to the above - places like Morecombe, Bishop's Auckland etc.
I'd also look at extending Merseyrail to Preston, Wigan and Helsby with a few more Battery 777s, to free a few more Northern Sprinters.
I haven't done the maths, but this feels like it could replace most of the Northern 150s and 155s either directly, or by replacing 195s to move to those routes. Any still left over would be replaced by 168s or 171s from Chiltern/Southern
The Northern 156s would be replaced by 158s from EMR and TfW. The EMR 158s would be released by TPE taking over Liverpool - Nottingham with their 185s, plus the remaining 168/171s.
By my reckoning, that should do the trick, without adding a single new diesel to the network, and minimising training and microfleets. Ramping up new electrification would be needed to phase out the 158/9s and 165/6s, but that's another story.