Lots of useful information above. Each to their own but I prefer a low-tech approach. A couple of years ago I walked from Swansea to Portsmouth using a combination of minor roads, the Wales Coast Path and the Kennet and Avon Canal towpath. Then I took a ferry to St Malo, followed river and canal paths most of the way to St Nazaire, coastal paths and minor roads south to Royan, ferry across the Gironde, and then the Voie Littoral to Bayonne. The final two days following the River Nive towpath and back roads to St Jean. Almost all navigation done with pages cut from a road atlas, backed up with screenshots of Ordnance Survey maps (UK) and IGN maps (France) on a smartphone for town plans and a few tricky junctions. Having the safety net of gps mapping and internet access on the phone is a great confidence booster though! However you do it - have a great experience!