The big jump in rail tech came with the power electronics and synchornous drives that came online in the 90s. You'd be looking at the drive gear from the Class 222 tuned down to 110 or 100mph and having 1 panto/tranny carriage per two gennie carriages. Each contributing to the DC Busbar with VSD invertors driving each traction package.
Since we know you can fit all this equipment (gennie, DC busbar kit, VSD and motors) in one 23m carriage fit for 125mph, and we know that you can fit the tranny and DC rectifier for the busbar into a 20/23m carriage. There's nothing technically stopping a 3 car DEMU for 110mph on leccy and 100 or 90mph on Diesel being knocked up.
You only really face the issue of ongoing Diesel supplies for the next 30 years, but there isn't any other kit mature enough for rail use, and without much effort, one could knock this kit together today.
Unlike the LDPE Diesel requirement that will likely be dead in 20 years with ongoing electrification, and can be provided for by hauling EMUs away from wires for their service patterns. EDMU Inter-Reigonal units will almost always be needed, and will likely de-marginalise additional infill for electrification, and increase the performance of Diesel services under wires as they'll be on the juice running at 1500kW rather than 1000kW or less on the Diesel.
Thus increasing the capacity on mixed traction lines like into Manchester, and equalising performance to aid in implemented ATO or Close Loop regulation to remove unscheduled stops and conflicts from routes in the form of an addition to ERTMS and ETCS...