If the go for the generator car option though then even if the initial sets are underpowered for running west of Newton Abbot then a high power generator car could be produced in theory. If they go for the underfloor engines option then you are stuck unless they change the engines.
"You can always build a bigger engine..." heh
But yeah, passenger comfort is an important consideration, so if we are going to have it with distributed traction, then having a seperate carrage with a sokking big engine in it would be most comfortable for passengers, basically the power can be supplied from either end carrage, one end using a transfomer from 25KV OHL, and the other with a massive engine. The underfloor engine option would greatly increase maintenance, as for an extended service to the engine, detach one and attach another, drive it to the north east for service, then swap it back again.
If they still want the 'Emergency get you home power source' that should be housed in the 'Driving Transformer Car' that will be standard between both bi-mode and electric only units, if that where say, a 1,000hp unit (not too big), a big enough fuel tank to keep it going as long as the generator on the other end when running off the wires then to balance with an HST (provided the aux engine in the panto/transformer is used aswell as the generator coach) it would need 2*2,250hp = 4,500hp in total. So, even with a Cummins QSK19 or similar under the floor of the Electric Transformer coach at 750hp, it would need 3,750hp at the other end to run as a 'lop sided HST with distributed traction' The Class 70 has a 3,690hp engine that fits in our loading guage, so fit an 810hp engine in one end, and one from a class 70 in the other end's full time 'generator car' then it will have the same amount of power as an HST. And provide emergency power for electric only trains.
If they can't fit the reserve power engine in the transformer car then it would have to live in the DVT to keep passenger comfort up, ie, not sitting on engines. And I'd bank on being able to find a single 4,500hp engine, or somone willing to build one, that will fit in our loading guage.