Well, they have ordered 140mph trains. It was the plan for the IC225s, and there's even a few sections of the current ECML signalling that were changed to flashing or double greens (can't remember which right now) for it, isn't there? In the end, they never followed through with it in the 90s, and we've run the IC225s slower than designed ever since.
I thought that ERTMS was going to finally enable it, since it seems like the thinking was that 125mph was as fast as we could go without in-cab signalling. Sure, the tracks and the wires may have to be maintained/upgraded for 140mph, but it seems like nearly all of the puzzle pieces are finally falling into place for it a mere 25 years later than originally planned (and probably 30 before we actually see it happen).