I use route 1 sometimes through London and one section is along the top of a dyke around some reclaimed land. It was a fairly good fine gravel path until motorbikes started using it but they churned it up and now the surface is loose rocks that were the path foundations, they have made it almost unridable.
Well that is a shame and would be very disappointing to see.
That is not the experience where I live, the main route is the Trans Pennine Trail. Most of it near me is extremely poor. Its used by horses, and probably mini moto's too, but I doubt it was the mini moto riders that buried half bricks and broken concrete hard core into the sand/gravel surface, or allow hedges and trees to grow uncontrolled until you cant see the path, or that designed tunnels without lights yet littered the floor are with bricks, glass and gravel. Unless your doing about 8mph its extremely uncomfortable to traverse and even a bit dangerous in places, turns your bike into a bone shaker and you get to your destination covered in grit!
The next route to me is the Middlewood way which goes from Marple to Macclesfield. Some of this has been tarmacced, some of it hasnt, and a very large portion of it is extremely loose large stone gravel basically just dumped on top by the ton. Again for all but mountain bikes in-traversable.
For me, these routes are nice for a sunny Sunday afternoon with the kids, but they are not really a national cycle network. A true NCN would have a good surface, be maintained and have things like drainage, you simply dont see that.
If we really had a good quality National cycle network, then I would agree, but what we have is laughable.