Stop / start electrification projects have been a disaster -- you lose expertise between projects, and that adds problems & cost to future electrifation schemes. So, spread over, say 40-50 years, I would pursue a Swiss style solution. Electrify almost everything, just omitting low-traffic rural routes. So it would be more a case of what not to electrify, and what had very low priority:
So, the following are examples of what would not be electrified:
Inverness - Thurso/Wick & Kyle.
Craigendoran - Fort William, Mallaig & Oban
Girvan - Stranraer.
Craven Arms - Central Wales - Llanelli
Hellifield - Carnforth
Battersby - Whitby
Par - Newquay
Low priority for electrification (for example):
Barrow - Whitehaven.
Grantham - Skegness
Norwich - Cromer / Sheringham.
Burscough - Preston
Looe & Falmouth branches.
The starting point for new schemes would be extending existing electrification areas and infilling gaps to eliminate most of the need for diesel working around big cities in England, Wales & Scotland.