@David-Ringland As general guidance, if you've got a lot of time, then ride down through Wales. It's quieter than England, the roads are less crowded, and quite often they're in better nick (not so over-used and potholed).
See the A483 - Welshpool, Newtown, Builth Wells, Abergavenny etc. It will get a bit sticky down near Chepstow/Bristol/Severn Bridge but that's the price you pay for the previous 100 fabulous miles!
Or before you get to Abergavenny cut across to Gloucester, and then you get a bit of Cotswolds afterwards.
Example route:
https://www.myrouteapp.com/route/open/8481843
2 = Crossgates - biker meeting place and petrol
5 = Avebury - like riding through Stonehenge, but free & with less people
If time is tight, then the A49 through the Marches is a good compromise. About 4.5 hours versus maybe 3.5 - 4 on motorways. An extra hour well spent.
Example route:
https://www.myrouteapp.com/route/open/8482009
I'm sure other users who know those areas can improve on the above routes, but they're not bad for starters.