We don't have working BEMUs yet. Would be nice, but with your suggested diesel engines, it would take many years for a working train to enter the market.
Meanwhile there should be a permanent ban on procuring any more straight DMUs. If a diesel-only unit is required it should be initially supplied as a DEMU, with passive provision for conversion to a BEMU, EDMU or pure EMU. I envisage that such a prohibition of diesel-mechanical multiple units (including those with hydraulic transmission) might be imposed in the near future, just like the mandating of 25kV capability on all mainline EMUs including those originally intended for DC only.
The procurement of the current DMUs has created the problem of diesel trains with projected lives beyond the likely cut-off date for pure diesel traction on UK railways. If DEMUs were ordered, with a 750VDC bus, they would have the same traction bogies as their EMU counterparts. Conversion to bi-mode EDMU would be a case of adding a pantograph and transformer with ac conversion electronics to power the bus and return regen. power back to the OLE. If a BEMU was required, the diesel gensets could be removed and battery sets fitted in their place with charging control electronics, as well as the ac hardware as for the EMU conversion. It would even be possible to have a bi-mode EDMU with battery mode as well if required. A common design would be capable of embodying all of those versions with considerable savings on maintenance and spares.
The current orders of diesel mechanical units are a result of a dead-end choice where changing them to any other type would probably not be worth the cost.
