All these sites are useful to find routes, get inspired and create new ones. Beware that some of them require a paid subscription in to order to download the gpx file.

Mapping 

• Strava
  Strong community but lately they have restricted a number of features for subscribers only. You can no longer new create routes. Your old routes can no longer be modified. This will be probably evolve overtime

• Komoot
  You need to have a subscription to download the GPX file. But probably one of the best for designing routes.

• Viewranger
  Great when you want to navigate using your phone. You can design your routes and download the gpx file for free too. This is a great one too. 

• Garmin Connect
  Easy to design routes and download GPX files

• Ride with GPS

• Trailfork
  Database for trails and their status. Not always up to date.

• Alltrails

• Wikiloc

• footpathmap
  shows footpaths, bridleways, byways

• bridleway maps
  shows bridleways

• Bike Map

• GPX viewer 

• Open Cycle Map

• Plot a route

• Gravel map

File merge

• Fit file tools - merges multiple gpx filesGX

• PX merged - 

Public right way

• Hertfordshire
• Surrey

Others

• Veloviewer 
  visualises your rides
• Epic ride weather
  helps plan the weather throughout your ride
• Cafe cyclist
  list of friendly cafes for cyclists
• Pitchup

Find a campsite anywhere in the UK