If you have mapping software I would think that's still the best method, if only for the weight: OS maps are heavy.
We did our own coast-to-coast walk of 183 miles and printed out our route at 1:25000 on 9 sheets of A4. Scaling up to your 1400 miles it would require 69 sheets, so that gives you a rough idea. It's quite a big initial effort to get it all printed but a lot lighter and more efficient than all those OS maps.
You could contact some of the LEJOG walkers like
Alan Sloman and ask how they did it.