If you want to use your hammock, avoid the Pennine Way at all costs!
The Ardennes are my stomping grounds; I can recommend the GR57 Liège-Diekirch. This summer I'm going to hike the GR571 Vallées des Légendes myself.
Both walks roughly follow rivers through forested hills and have a lot of points along their routes that are accessible by train. You can also link some GR's together.