We've heard that Bombardier and Alstom aren't interested in DMUs any more. Stadler and CAF are, but their products are regional 100mph sets. Because the line between commuter, regional and express EMUs has blurred it really isn't that difficult for a manufacturer to turn their 100mph design into a 125mph design. Modern crash regulations allow traditionally designed sets to run at up to 115mph, I believe. With all of the electrical systems designed for that sort of top speed, increasing it by another 10mph isn't hard, and then all that needs done is for the body structure and layout to be tweaked for the increased crash safety requirements. As far as I'm aware, all of Bombardier, Siemens and Hitachi will sell 125mph versions of their standard regional EMUs.
For CAF or Stadler to up their 100mph DEMUs to 125mph it would be a lot more complicated. Power requirements increase with the square of speed, so engines must be larger. However, they also need to have all sorts of modern emissions filtering systems which take up room, so they get very large very quickly. Hitachi have demonstrated this by having to raise the floor above the engines in their AT300. For CAF to build a 125mph version of the 195s for Northern, they would have to go around changing a lot of things. Of course, there's no reason why they couldn't do this, but it would cost money. Remember that since the IEP process began and electrification of the GWML and MML was confirmed, all of the train manufacturers will have assumed that there would never be additional 125mph pure DMUs built. If any more sets were required, then Hitachi would be the only manufacturer capable of producing them at an acceptable cost.
On top of that, the costs will increase further because the banks which finance the orders of new trains won't want to buy something with an inherently short expected life. We already have 125mph pure DMUs - the 180s - but they're already in a fairly precarious position as they have no real point when most high speed lines are being wired up. Hitachi sets have a guaranteed lifetime ahead of them, as the demand for 125mph EMUs will never go away. If the engine sets are leased from MTU rather than bought by the ROSCO, then there's basically no risk involved in financing them, so the ROSCO can offer a cheaper leasing deal.
If for whatever reason Hitachi bi-modes just could not work on the MML, then the alternative seems to me that they could order them to cascade other straight DMUs onto the MML instead. Grand Central, for instance, could give up its 180s and take advantage of the wires and the Hitachi experience and servicing available on the ECML, as could CrossCountry giving up some of its 22x trains.