I live in Kendal and would strongly recommend avoiding the A591 during the daytime. A reroute from carnforth to the A6 north of Kendal is possible avoiding the really busy roads and avoiding adding significant distance. If your not familiar with the area taking the Burton road A6070 and then the A65 through Crooklands is the easiest way. Even nicer routes are available but increases Nav requirements.
Would also recommend taking in Hutton in the Forest and Durdar between Penrith and Carlisle to avoid the rolling schlep on the A6
As other have said thanks for the suggestions, I had planned the first suggestion by looking at the map prior to you mentioning it based on previous experience, the A6 between Penrith and Carlisle I have ridden before but it was at silly o'clock so no traffic so thanks for the suggestion I have included your advice in my plans.
I have split my ride into 4
324 kms day 1 to Paisley
421 kms day 2 to a wild camp somewhere near Fort Bill on the return.
402 kms day 3 with a stop off at Paisley
106 kms day 4 mostly downhill and easy on the head.
That's the plan lets see how it goes.