The 40 miles between Cambridge and King's Lynn only carries 1tph electric off-peak.
But, disregarding bi-modes, electrifying Cambridge to Kings Lynn avoids having to run a diesel all the way into London. Most of these are also lengthened by splitting and joining at Cambridge so either the strengthening trains would have to be diesels too, or diesels and electrics would have to run in multiple. Hence, as per my previous point, Cambridge to Kings Lynn had a good case for electrification despite a low service frequency.
For a number of years, the WCML north of Preston/Lancaster only carried 1tp2h Euston-Glasgow.
The Manchester and Birmingham trains (and most of the freight) had reverted to diesel.
Much the same applied to Weaver Jn-Liverpool before the 350s arrived.
Stafford-Wolverhampton and Carstairs-Edinburgh had virtually no regular electrics at all.
When the electrification was inaugurated in 1974, nearly all trains north of Preston were electric. Economics changed subsequently due to factors such as multiple units replacing locomotives, and the reduction in total freight and split between operators making it uneconomic to have spare locos and crews to allow electric/diesel loco swaps on freight trains. If electrification north of Preston had not been done in 1974 and was being contemplated around 1990, it might have been difficult to make a case for it. However with two to three electric passenger trains per hour and a proportion of the freight being electric, it would probably be worthwhile to electrify the WCML on today's criteria.
Cardiff-Swansea may only carry 1tph electric off peak (the reason its business case is poor).
Much more tricky to justify especially as the 1TPH is a bi-mode, there's very little freight of any sort and what runs would be difficult to convert to electric due to running west of Swansea. It would help if the other 1.5TPH were electric too, but that would involve severing the through service via the Marches.