@minininjarob if you haven’t set off yet consider these.
Train to Appleby in Westmorland or Penrith and follow the Pennine Cycleway to Berwick. Lovely ride and only one, Hartside, serious climb that I can recall.
Another would be to pop over to Lancashire to ride the Lancashire Cycleway or parts of it. It’s +/- 270 miles in total. A good combination of flat, rolling and hills.