The biggest challenge in planning this trip has been to comb through the vast amount of information available for the different sections of this trip and condense it all down into a comprehensive and lightweight version.
There are two guidebooks on the market that feature the whole route - and hundreds of guidebooks and mapsets that cover sections of it. One of the "thruhiker guidebooks" by Andrew McCloy turned out to be rather useless. It contains only route descriptions without any actual maps. I was already starting to get discouraged by all the confusing route descriptions when I luckily came across the Cicerone guidebook by Andy Robinson "The end to end trail". This was exactly the kind of guidebook I had been looking for: It describes the whole route in good detail and gives sketch maps for the sections that are not part of a waymarked long-distance trail. It even gives overview information on shopping opportunities and accommodation. This book has proven to be invaluable in the planning process (I will almost completely follow its route suggestions) and will definitely be the main guidebook on trail.
Another invaluable online resource turned out to be the website of the Long-Distance Walkers Association. Their searchable data base does not only show all British long-distance trails on a map but also shows where they connect with each other. There is a lot of information for each trail and you can download gpx tracks for most of them. All the info is free, but you have to be a member (13 £) to download the tracks. (I found gpx tracks for all the trails I needed for free elsewhere on the net.)
I wanted to create a gpx track for the whole route and use the track together with maps on my GPS. So next I needed GPS maps for the UK. I was even willing to spend many on that, but everybody advised against the Garmin topo maps for the UK: Too much money for bad quality. There are a lot of other GPS maps for the UK on the market, but they were all for route planning only and could not be downloaded onto the GPS - and of course they were expensive, too. The solution came with Talkytoaster.info: Here you can download openstreetmap based maps for the UK - completely free! I was a bit afraid of the downloading process that sounded rather complicated, but it all worked fine. Mind you though that these maps are ok when following an existing gpx track, but they are by far not detailed enough for route planning of hiking trips.
Now I needed good online maps to map the missing sections based on the sketch maps in the Cicerone guidebook. The solution was www.grough.co.uk. Grough turned out to be a mixed blessing: First of all it is very cheap. 2 £ buy you one month of unlimited mapping using the wonderful Ordenance Survey maps and 40 pages of print outs. The map details are fantastic - 100% like your typical OS paper map and the printing is easy and renders high quality results. But grough is not the perfect solution either: First of all you can only use these maps online - you cannot download them onto your computer or your GPS. This means that everything takes forever. Theoretically you can upload existing gps tracks and print them out on the respective maps. Unfortunately, this did not work at all. Uploading tracks that had more than about 50 trackpoints led to constant time outs and system crashes. I got very frustrated and wasted entire days with this system until I realized that it is much faster to create the track in grough itself instead of unsuccessfully trying to import it. Still this process was very time-consuming, but less frustrating. Exporting tracks on the other hand worked quite well as long there were less than about 300 waypoints per track. I also used grough.co.uk to print out maps for the connecting stretches.
So now I will use my Garmin etrex GPS with OSM maps and a gpx track for the whole route. I have print out paper maps from grough.co.uk for the connector sections and will buy guidebooks or stripmaps for the established long-distance trails locally. I will carry the Cicerone guidebook that shows the connector sections in sketch maps.and has route descriptions and town info. And last but not least I have an info sheet with all the logistical information on store hours, cheap hostels, gas canister availibility etc. that I have compiled myself with a lot of internet research.
A lot of work has gone into this project and I do hope it will pay off.