The 91 would not be passed for IEP speeds. I don't think the drivers would accept that anyway. You are also looking at extra crewing for the attatch/detatchment of locos & all that goes with that type of antiquated working. Our railway & its infrastructure don't lend itself to that these days.
You also have to consider the lease costs of the loco(s) which is likely to be significant, in which case the costs associated with a tilting bimodal train would probably start to look like good value.
I don't know how much a loco costs to lease, but let's say it's £300,000 a year and say you need two of them over a 15 year period that's £9 million (broadly the length of time that 221's could well so be in use, due to electrification or more likely needing to be replaced due to age).
Add in a 1,000 miles a day of track access charges and even at the lowest loco rate of 45p a mile that's another £2.5 million over those 15 years.
That's enough that even if you'd be paying 10% more in lease costs for tilting bimodal trains that it would still be better, even before you consider fuel costs (due to more efficient engines and a lighter train when under the wires which would need less electricity to move, and before anyone says anything about dragging fuel around you'd still be doing that with the 221's) extra maintenance costs, etc.
Then there's the fact that modern trains appear to be cheaper (at least in terms of maintenance costs) and so even with the extra costs of developing, designing and providing a new tilting bimodals train compared to current new trains they could still be comparable to the existing units and certainly cheaper than doing long electric loco drags from London to Crewe.
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