Either of the attached is nice. Well, nice by my standards anyway. Lumpy-ish and scenic. I'd probably favour the southern of the two, with a variation on that being to take the road up to Tan Hill from Reeth, rather than along Swaledale, and drop into Kirkby Stephen via Nateby.
I can't vouch for the first section of the first one, from Scotch Corner to about half way to Barnard Castle, but the rest of the roads are good. The quieter route is the southern one. The road from BC to Middleton and beyond is not busy as such, but it's busy relative to most of the others in both routes.
I'd not want to cycle on the A66 at any point between those two places. It's tolerable, just about, beyond Penrith, but otherwise very busy and very fast.
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