If you’ve got an old man pass, then Carlisle or Berwick are ideal start/end points. A couple of thoughts off the top of my head.
From Carlisle you could do a great wee loop of the various dales and hill climbs through the N. Pennines, heading east (via Brampton, Lanercost, here riding along Hadrian’s Wall to Gilsland and then Haltwhistle) picking up the S. Tyne, over/down Allen back into Tynedale, Derwent, Weardale, Teesdale, over to Brough, then follow the Eden (Hilton, Dufton, Knock, Kirkland, Melmerby… (trip to UK’s highest tarmacked summit/longest road climb optional)) then Gelt (Castle Carrock, Warwick Bridge) and back to Carlisle. As long as you want each day if you travel the best ways, but scope to ease the workload and sightsee if the hills are too much.
Various cycle routes to be followed along the way, and a great variety of landscapes and riding. History, scenery, solitude, scattering of cracking pubs, towns and villages, and some of the UK’s best (and quietest) climbs and roads. Only downside if you’re doing it in the near future is possibility of snow/ice – if that’s the case head to the Lakes where roads are lower and generally snow free.
From Berwick, head through Northumberland. Duddo, Etal, Ford, Wooler, Ilderton, Ingram, Alwinton, Harbottle, Elsdon, E/W. Woodburn, Bellingham, Wark, Simonburn, Warden, Corbridge, Matfen, Kirkharle, Wallington, Rothbury, Alnwick, Alnmouth, Howick, Craster, Newton, Embleton, Seahouses, Bamburgh, Belford, Fenwick, Beal, Holy Island, Beal (again) inland to skip the A1 or if you’ve tough tyres follow the coastal path for the last miles to Berwick. Again, options for detours and shortening the days as and when you like.
As the above, follows various NCN routes. Skirts cheviots and high ground at the start, crosses Hadrian’s wall before heading back north via the coast. Like the other route, assorted history, quiet roads, good pubs and a mix of scenery/landscapes. And again, quiet roads in the main.