Good point.
This is a problem with some of the electrification wishlists I've seen. No disrespect to the good people of East Anglia, but there are some branches run by 153s where a full sized EMU would be a bit of a waste.
Then again, if you think that the "main lines" in East Anglia have a good enough case for electrification then there's the question about what to do with the small number of DMUs left over. Could it be more economical to electrify the branches than to retain a handful of 153s? Especially as electrification allows through running. But I can't be sure.
I'm pretty sure that if you put out an OJEU tender notice for two car EMUs, the usual suspects would respond with suitable designs from their standard range, with the equipment divided up and packaged suitably.
Single carriage EMUs exist in Europe, so I don't see any reason to believe they could not here
Wouldn't the point of this be to run Norwich trains to Kings Cross, which has a much quicker journey time to Cambridge than Liverpool St?
Capacity through Welwyn would be the problem there, plus the fact that it's further from the City (although convenient for St Pancras). Upgrading the Hertford Loop might help, although the Lea Valley would be preferable, perhaps absorbing the King's Lynn trains and giving two more Welwyn paths.
could do Kings Cross without using any more paths, though it would need additional stock- simply extend the Cambridge Express/Cruiser/whatever it is called that terminates at Cambridge to Norwich, in the same way as the opposite half hour extends to Kings Lynn.
On the WA, I reckon if you could run the off peak "fast" as it does in the peak- Whittlesford, Audley End, Bishops Stortford, Tottenham Hale- that would give an acceptable journey time. You're not going to be able to a non-stop like on the Kings Cross line.