I agree with the suggestion that an electro diesel is the way forward. We are going to have many parts of the country where most of the lines are electrified particularly on the congested lines where smart working is required. Most of the none electrified lines in these areas will probably be low speed and have a limited service so lesser performance on diesel would be acceptable. Only in the remoter parts of the network would pure diesels be required and this could be done for many by cascading units replaced by electro diesels elsewhere. The class 92 is a 20+ year old design and I am sure they could do much better now. On the continent there are many areas where most lines are electrified so the potential market could be large.
While the TRAX design is attractive it is little more than a low powered shunter off wire. However it is designed with a low frequency (16.7Hz) transformer, which if replaced by a 50Hz only version could save as much as 10 tonnes of weight allowing a larger diesel and fuel tank to be fitted. The maximum speed could be limited to say 90mph and the power to 4MW more weight could be saved. It could then become a good class 20 replacement for all the odd duties around the system such as light freight and maintenance trains.
Another design that could be considered is a Bi mode class 37 replacement CoCo design. Keep the weight down to about 110 tonnes and a top speed of a 110MPH, an AC power of 5MW, 3MW on DC and maybe 1 to 1.5MW on Diesel (with something like an HST engine) and you would have a useful general purpose locomotive. It would have enough power to work substantial distances off wire, could go anywhere on the system and could be used for freight and Passengers.
Finally a heavy haul Electro diesel could be considered (a bi mode class 70). With high axle loading and high tractive effort so the maximum speed would have to be limited to maybe 70MPH. But then a substantial diesel engine could be fitted maybe up to 2MW rating as well as 5 or 6MW on AC.