The problem is, as with everything else in this country the cost is ridiculous; electrification and HS2 cost per mile is madness compared to our French, German and Spanish counterparts. We also have a clown running the Department for Transport who claims bi-mode is "just as good" as electric. The Great Western, Transpennine and Midland should've been electrified years ago. I'd say other than that, the pressing next phrase is the Chiltern & Snow Hill and infills/branches like the Uckfield, North Downs, Felixstowe, Middlesbrough/Sunderland, Hull, Wolves-Shrewsbury, WCML-North Wales and West of England ML. That would take a load of diesel trains off the electric routes. Running diesel trains on the WCML just so it can travel between Chester and Crewe (20ish min journey) is madness.
I think the list of priorities should go like this:
1) Major mainlines: i.e. Kettering-Derby-Sheffield, East Midlands Parkway/Long Eaton-Nottingham, Nottingham-Ilkeston-Chesterfield-Sheffield-Barnsley-Normanton-Leeds, Sheffield-Doncaster-Cleethorpes, Swinton-Fitzwilliam, York-Leeds-Huddersfield-Guide Bridge, Micklefield-Hull, Doncaster-Selby, Deansgate-Liverpool South Parkway, Reading-Bristol Parkway-Cardiff Central-Neath-Swansea, Royal Wootton Bassett-Chippenham-Bath Spa-Bristol Temple Meads, Reading-Bedwyn-Taunton-Tiverton Parkway-Exeter St Davids-Newton Abbot-Plymouth, Taunton-Bristol Parkway-Gloucester-Derby, Crewe-Chester-Llandudno Junction-Bangor-Holyhead, Warrington Bank Quay-Chester, Stockport-Hazel Grove-Chinley-Sheffield, Haymarket-Aberdeen, Polmont-Perth, Croy-Larbert, Perth-Inverness
2a) Major branches of those lines: i.e. Oxenholme-Windermere, Lincoln-Newark Northgate, York-Scarborough, Leeds-Halifax-Brighouse-Huddersfield, Northallerton-Middlesbrough, Eaglescliffe-Sunderland-Newcastle, Hull-Bridlington, Newport-Gloucester-Swindon, Shrewsbury-Wolverhampton, Hereford-Bromsgrove, Kidderminster-Ashchurch for Tewkesbury, Newton Abbot-Paignton, Inverkeithing-Markinch/Thornton, Stirling-Alloa
2b) Commuter lines: i.e. Welsh Valleys, Chinley-Ashburys, Rose Hill Marple branch, Buxton branch, Stockport-Greenbank, Bolton-Clitheroe, Leeds-Harrogate-York, Manchester-Southport, Manchester-Kirkby, Manchester Victoria-Halifax, Sowerby Bridge-Brighouse-Mirfield-Wakefield Kirkgate, Woodlesford-Knottingley-Doncaster, Wakefield Westgate-Pontefract Monkhill, Hatfield & Stainforth-Gilberdyke, Sheffield-Retford-Lincoln, Nottingham-Worksop, Belper-Matlock, the Chiltern Mainline, the Snow Hill Lines, the Chase Line, Nuneaton-Leamington Spa, the Stourbridge branch
3) Other regional lines; e.g. Newport-Shrewsbury-Crewe, Shrewsbury-Chester, Borderlands, Water Orton-Norwich, Harwich, Felixstowe, Clacton, Walton, Lowestoft, Yarmouth, Lincoln-Barnetby, Newcastle-Metrocentre etc.