Navigation shouldn't be a problem at all - there are the National Trail routes to follow and Centurion put out red and white tape as well. You are also required to carry a map, so no, not reccie-ing the whole route will be a problem - I hadn't reccied any of it.
There are a few blogs about it that you may want to read
You could also incorporate the Ridgeway if you go up the Thames path, then join the Ridgeway at Goring, but I'm not sure where you'd turn off towards Calne as I don't know the area west of the Ridgeway.
What Joolska says! Good for you Tim for working something out for yourself (I'm more of a sheep and want people to organise for me!) - maybe get the route onto Social Ultra and you may get others to join you - if you wanted company that is?
I'll be like T-Rex - even my wife likes watching those orange flags moving about!
Scotland is much more relaxed about its camping and right to roam, marvelous place!
booktrunk - stay away from people with colds - you don't want another knock-out blow like you did in October. You should be like Bradley Wiggins - avoid toilet doors, don't shake hands with people etc! Great to hear you're so excited, it'll be some adventure - difficult and extremely rewarding. It'd definitely on my bucket list, but I'm thinking 3-5 years time realistically.