Looking to do this in Sept with a few friends. We would like to do this in 3 days with as much off road as possible.

Can anyone offer any advice please?

Trans Pennine trail is coast to coast if that help?

We did a 3 day one with about 50% off road, our route was Whitehaven to Penrith, Penrith to Allenheads, Allenheads to Whitly Bay. You could probably plan your route to have more off road than we did, but I think we had a nice mix and all off road would increase your saddle time significantly.

A bunch of us did this recently,mostly on road and cycle tracks,if you get the Sustrans sea to sea map it shows a lot of alternative off road route,will be interested to hear how you get on as i would like to do the same.