Even a small alteration would require all the tolerances, thicknesses and components to be redesigned. Which would then be a brand new design requiring design, prototyping and testing.
You cant just switch from a aluminium body to a steel body, lengthen it by 3m and move the bulk of equipment under floor and say it would require minimal design work! Theres also the requirement the Pendolino would have to meet newer energy efficency and weight requirements since it would be a new design rather than an old design.
Why would you use a steel body? What is wrong with the existing aluminium one? And unless you are seriously suggesting it would be heavier than a train with tonnes upon tonnes of diesel engines under the solebar, I doubt it would have trouble with weight restrictions.
And even if you did have to redesign various components, the really expensive part of a design is the electrical systems, and those would be identical to existing Pendolinos, whether 390s or continental ones. Energy efficiency targets would be easily me thanks to drastic reductions in bogie weight since they would be lightweight types rather than tilting ones.
The meridian doesn't seem to have required a massively expensive redesign programme, unless they seriously carried one out for a run far smaller than this one would be.
The Dft figures giving 2.2m for IEP electric, 2.7m for lengthened Pendolino and 2.8m for IEP bi-mode carriages are quite interesting.
It raises questions as to why the bi-mode is only slightly more expensive to lease than teh electric, it just looks like another badly designed bloated PFI.
EDIT:
As to the work requirement to lengthen an existing design, if it is so insanely hard why did BR even consider attempting to lengthen the Mark 4 to 25.5m apparently after the order was quite advanced?
And the figure would likely be closer to 6ft than 10ft, thanks largely to the fact that Pendo carriages are already 23.9m long.