That's quite a distance! You will need some good maps.
Sustrans is an excellent source. They have a John O'Groats to Lands End series that might be helpful.
Why is it you are only camping in Cornwall? Tenting in Scotland might be easier as the land is not as populated. As to how to carry a tent, they are usually carried on a rear pannier rack and are quite small and lightweight. I use a camping hammock myself.
As to security; I carry anything of value with me, so I never fear leaving my bike and panniers. I do have a couple of stout locks and an alarm just to discourage thieves. The panniers have little locking devices too.
I've never toured in Scotland but have toured extensively in the southwest of England. Check out my diaries and get back to me.
Good planning is the best insurance for a successful tour. Start with realistic expectations and be flexible if your plans are not working. Touring is as much fun as you can have without actually smiling. That said I've been known to smile out loud on some on the better days. Even a bad day touring is a better day than a good day when you are not.