Here's my two groats, and I'm presuming quite a lot re ideal province size/function. All county references are pre-74. I'm making this up on the hoof so please correct or rubbish below at will.
ENGLAND
??CORNWALL - Cornwall, cap Bodmin
DEVON - Devon, cap Exeter
SELWOOD - Somerset and Dorset, cap Taunton
WESSEX - Hampshire, Wiltshire and Berkshire, cap Winchester
??SUSSEX - Sussex, cap Lewes
KENT - Kent and Surrey, cap Canterbury
ESSEX - Essex and Middlesex, cap London
CHILTERNS - Oxfordshire, Bucks, Herts and Beds, cap Oxford
EAST ANGLIA - Norfolk, Suffolk, Cambridge area, cap Norwich
??MIDDLE ANGLIA - Northants, Rutland and Sokesey, cap Peterborough
HEREFORD - Gloucs, Hereford, Monmouth, cap Gloucester
CHESHIRE - Shropshire, Cheshire, Flint, Lancashire below the Ribble, cap Chester
MIDLANDS - Staffs, Warks, Worcs, cap Warwick
THE BOROUGHS or maybe SHERWOOD - Leics, Notts, Derbys, cap Nottingham
??LINDSEY - Lincs, cap Lincoln
CUMBERLAND - north Lancs and Cumberland and Westmorland - cap Carlisle
YORKSHIRE - tough one this, as it's massive, but it does and should dominate the region. Whole of Yorkshire, cap York.
DURHAM - Durham, cap Consett, er, Durham.
NORTHUMBERLAND - Nld, cap Alnwick
??TYNEDALE - upland parts of Northumberland, cap Hexham
WALES
GWYNEDD - 'Gwynedd' and Cardigan - cap Caernarfon
POWYS - 'Clywd' and Montgomery and Radnor - cap Llangollen
DEHEUBARTH - Pembroke, Carmarthen, capital Carmarthen or maybe St Davids (Mynwy)
GLAMORGAN - Everything else, inc Brecon but exc Monmouth, cap Cardiff (or Llandaff?)
SCOTLAND
MAN - Man - cap Peel
GALLOWAY - Galloway (only) - Wigtown, Kyle and the Rhins - cap Whithorn
NITHSDALE - Lanarkshire, Dumfriesshire, Kirkcudbright - cap either Dumfries or Lanark
MERSE - The western Borders, including most of the unit imaginatively dubbed 'Borders' by the cunts that butchered the old counties - cap perhaps Roxburgh, perhaps Selkirk, perhaps Dunbar, Melrose?
LOTHIAN - The south Forth region, including Edinburgh, cap Edinburgh

Also a slice of the area to the south to prevent clutter.
STRATHCARRON - Stirlingshire, judiciously expanded westwards to merit province status, cap Stirling
ALBA - Emotively termed stopgap solution to name for region between the Tay and the Mounth. Capital either Perth or Scone.
FIFE or STRATHEARN - Fife and a slice of the hinterland, to include all not allocated above. Cap Dunfermline
??ARGYLL - County of Argyll extended northwards to include everything on the Scots west coast south of Loch Linnhe. Cap ??Oban.
SOUTH SODOR - All offshore islands north to and including Mull, plus Kintyre, cap Finlaggan
STRATHCLYDE or LENNOX - Clydebank and Clydesdale. Cap either Glasgow, Renfrew, Dumbarton or Largs.
MAR - Aberdeenshire plus everything west of the Spey. Cap probably Aberdeen.
MORAY - Between the Spey and Loch Nes. Cap likely to be Forres. (I would love Cawdor

)
ROSS - Between the Great Glen and Caithness (with its anchorage on the west coast). Cap Dingwall.
CAITHNESS - Ultima Thule. Cap Wick. (Norse Katanes)
NORTH SODOR - All offshore islands between South Sodor and Orkney. Cap Dunvegan.
ORKNEY - I wonder whether Orkney and Shetland should be a unit. At any rate, the capital must be Birsay.
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Hmmm, this gives Scotland 16 provinces and England 'only' 20. Perhaps we should merge Strathearn/Alba, North/South Sodor, Moray/Mar, and Strathcarron/Argyll. Perhaps Man should be considered part of the province of Sodor.
But sod it all for now, I'm off to bed.