My maps have arrived from Sustrans - showing the cycling network between South Wales and London. This is largely what I'll be following.
The wonderful news is that between Bath and Reading, there is a canal which has a clear footpath run along it. This is the route I will follow across England (and there's even a few shortcuts!). Over the next few days I can properly work out the route, including the stopover points. That means I can start booking hotels and B&Bs!
