Regarding routes through France to Basel and Kandersteg, there are several options.
Eurovelo 5 goes from London to Calais and over Brussels and Luxemburg towards Basel.
If you like a more quiet and direct route with a theme, take the
WW-1 Frontier route, that runs from Nieuwpoort (near Calais) over Reims and Verdun to Basel. There is a dutch guide that describes this route in detail.
From Basel to Kandersteg you can follow a combination of
Swiss Fernradwege (long-distance cycle routes) or make a more direct cycle route using Google Maps.