I only do paths as well OP, never roads, or very short sections or we walk it
Peak district is amazing
Monsal Trail
Manifold Way
High Peak Trail
Tissington Trail
Then you have coastal routes, round south Wales, I havent done this yet
Bristol to Bath, I havent done that either
Morcambe bay, very nice
Exe esturary trail, this is lovely, down the Exeter canal
Round the canals between Edinburgh and Glasgow, we went to the bit near the Kelpies and the Falkirk Wheel, amazing, fantastic
The Midlands is really good for old railway lines, Northampton etc
Also Dorset for the Portland trail around Weymouth, thats really nice, The North Dorset Trailway and then the Castleman Railway