In reply to Liam M: You may be thinking of
http://www.magic.gov.uk - it's actually a tool for local authority planners and the like. Click on Interactive Map, select something like Administrative Areas from the "what do you wish to view?" list, enter a postcode, grid reference or whatever and the map should appear (you'll need to disable your pop-up blocker if you have one). You can measure distances using the area measuring tool - it's the ruler with the dotted loop above it in the Map Tools tab. The pop-up window shows the length of the "perimeter" as you trace your route on the map. It's better than the tools based on Google Maps (eg mapmyrun, runstoppable) for routes in open country but you can't save routes (that's not what it is intended to be used for anyway) and it doesn't show elevation profiles. The Google Maps tools can be OK for routes in open country if the satellite photos of the area in question are sufficiently high resolution.
