Functional bikes. Not Porn not Anti

Is there some network of biekable but rugged paths in this country that this kind of bike is suitable for?

I see these bikes (LHT, AWOL, Fargo, Vaya, touring setups of things like the Surly Ogre) and I think they're too heavy and overengineered to enjoy riding on roads for long periods so where would you ride them (in the UK)? Farm tracks?

Plenty of bridleways up north. Peaks and Pennines provide plenty of interesting tracks and trails with some mild elevation thrown in for kicks. Then there's the Lake District, Wales and Scotland - plenty of off roading around there. Towpaths make for a more relaxing day out or covering a lot of distance avoiding traffic.