Thank you to everybody who contributed to this thread.
I've edited the first post to reflect what seems to be the final consensus, and to say the list is now frozen and I won't be updating it further.
A very interesting discussion: gratifying to see that many of the busiest sections of unelectrified lines are due to be covered by TRU and MML electrification, and most of the rest could be addressed by extending existing schemes (Meadowhall, Filton, Bradford). Which leaves the Chiltern mainline as the main standout, and places like Hull and Exeter which look liike prime candidates for electrification islands for BEMUs.
Thanks again all.